Greetings! This is certainly an interesting question in the field of imperial history and it beckons us to consider how, in the "great game of empires", such smaller European powers were able to maintain far-flung colonies without the threat of the larger empires simply taking them over. This response will focus mainly on the 19th century and the "high water mark" of European empires, when all of the countries mentioned in OPs response were engaged in the consolidation (and at times expansion) of their imperial possessions. This response shall also view this question through the lens of geopolitical relations, rather than a colonial or indeed, sociopolitical approach. Let's begin.

Prior to the Napoleonic Wars and the advent of an industrial age, the empires of Europe were very much in active competition with one another. The British and French had clashed in what would become Canada, and the Seven Years War (1756-1763) was fought in battlefields as far as the Indian subcontinent and in North America. Peace deals often included colonial concessions from one empire to another, and it was not uncommon for naval squadrons to encounter each other hundreds (if not thousands) of miles away from the "mother countries" back on the Continent.

Here, the Portuguese and Dutch empires were keen to defend their imperial possessions, and in the case of the latter, prevent British attempts to takeover their Pacific territories. In fact after the Napoleonic Wars, the Dutch lost the Cape Colony to the British, which would (by the end of the century) become South Africa. Portugal, for its part, was spared these attempts by the British due to an alliance stretching back to the 1370s and 1380s, argued by many historians to be the longest-standing alliance Britain (or back then, England) possesses. As a result of this Anglo-Portuguese Alliance, the British had agreed not to infringe upon the colonies of the Portuguese Empire, though that did not stop it from extracting concessions elsewhere. For example, after the Portuguese court fled to Brazil following Napoleon's Iberian campaign, British aid to the Portuguese government was granted at the price of opening up Brazil's ports to trade in 1808. With Britain as a "shield" of sorts, the Portuguese empire continued its existence (and decline) as the 19th century wore on into the 20th.

Alongside this curious state of affairs between the British Empire and Portugal, the end of the Napoleonic Wars in 1815 also saw the birth of a new "system" of international relations in Europe: the Concert system. Under this system, the Great powers of Europe (Britain, Austria, Prussia, Russia, and France) would meet together under times of international crisis to mediate a solution which benefitted all parties involved and maintained the status quo of the situation. It was, in essence, a commitment to maintain the balance of power within the continent and beyond. As a result, European wars subsided in frequency (with the major exception of the Crimean War in 1853-56), and the 19th century was instead marked by "colonial conflicts" within empires, seeking to consolidate or expand their borders. It was by no means a perfect system, and faltered after the middle of the 1800s, a result of tensions between the Great Powers themselves, and of domestic concerns (especially within the United Kingdom) of maintaining such "commitments" to the Continental System of diplomacy. But these shortcomings do not detract from the overall value of the "Concert of Europe" for the relations between the European nations. Richard B. Elrod on what the system enabled:

"It [the Concert] was, after all, primarily a vehicle (and a fairly successful one) for the peaceful management of great-power rivalries. Above all, the Concert idea was realistic in the best sense of the word - attuned to, but not narrowly bound by reality. At its worst, the Concert was an impotent assembly, merely adhering to the formalities, unable to resolve important and pressing issues. At its best, it represented a reasonably satisfactory solution to the most difficult problem of international systems: how to accomodate the forces of change and yet preserve peace and stability. Concert diplomacy allowed the great powers to sanction necessary alterations of the existing order; it provided the means of legitimizing change without endangering the general system."

In addition to these benefits, the Concert system did manage to safeguard the imperial possessions of the "smaller nations" (Portugal, the Netherlands, and later Belgium) from conquest by larger powers. Any actions by a single Great power to forcefully take such territories would likely spark backlash from the other four, which might have been enough to cause them to back down from the whole venture. In addition to this larger "umbrella" reason, we might also consider the general observation that statesmen in 19th century Europe were more concerned with the security of their own countries than capitalising upon the weaknesses of other nations. In an age beset with the rise of "revolutionary" movements and the seeds of independence movements being planted across the colonies, the Great powers of Europe were not particularly open to the idea of invading the territories of the 'lesser powers' for more colonial troubles. The Dutch empire for example, had already been beset with various "revolutions" in its own imperial possessions from the late 18th to early 19th century, in some cases necessitating "temporary British control" (or in others, such as the case of Sri Lanka, a more..longstanding cession).

So to summarise, in the 19th century, the smaller European countries' empires were able to be maintained and preserved (at least from other European nations) through the larger considerations of the Concert System, as well as the prevailing mood of national security and continued prosperity over costly and messy imperial expansion. Hope this response helps, and feel free to peruse the source below (if accessible) for some more specific case studies of how the Portuguese and Dutch empires were able to be controlled in the first place.

Sources

Bethencourt, Francisco, and Geraldo U. De Sousa. "The Longevity of the Portuguese Empire: Problems and Hypotheses." Mediterranean Studies 9 (2000): 17-34. Accessed April 4, 2021. http://www.jstor.org/stable/41166908.

Darwin, John. The Empire Project: The Rise and Fall of the British World-System, 1830-1970. Cambridge, UK: Cambridge University Press, 2014.

Darwin, John. Unfinished Empire: The Global Expansion of Britain. New York, NY: Bloomsbury Press, 2013.

Elrod, Richard B. "The Concert of Europe: A Fresh Look at an International System." World Politics 28, no. 2 (1976): 159-74. Accessed April 4, 2021. https://www.jstor.org/stable/2009888.

Oostindie, Gert, and Bert Paasman. "Dutch Attitudes towards Colonial Empires, Indigenous Cultures, and Slaves." Eighteenth-Century Studies 31, no. 3 (1998): 349-55. Accessed April 4, 2021. http://www.jstor.org/stable/30053668.

Oostindie, Gert. "Dutch Atlantic Decline during “The Age of Revolutions”." In Dutch Atlantic Connections, 1680-1800: Linking Empires, Bridging Borders, edited by Oostindie Gert and Roitman Jessica V., 309-36. LEIDEN; BOSTON: Brill, 2014. Accessed April 4, 2021. http://www.jstor.org/stable/10.1163/j.ctt1w8h3c9.18.

Prestage, Edgar. "The Anglo-Portuguese Alliance." Transactions of the Royal Historical Society 17 (1934): 69-100. Accessed April 4, 2021. https://www.jstor.org/stable/3678521.

Viaene, Vincent. "King Leopold's Imperialism and the Origins of the Belgian Colonial Party, 1860–1905." The Journal of Modern History 80, no. 4 (2008): 741-90. Accessed April 4, 2021. https://www.jstor.org/stable/10.1086/591110.