If we're equating size--"the largest empire of all time"--with success, then there are basically four things that I think make the difference: the United Kingdom's financial power, created through the "fiscal-military state" in the 18th century; industrialization and economic growth; Britain's population growth and Britons' propensity to settle overseas; and India. Let's begin with ruling out a couple of things, though.

First, it's obvious why the British developed a larger empire than the Dutch over the long term: there are a lot more Britons and they had access to a considerably greater domestic resource base than the Dutch. Britain is several times the size of the Netherlands, giving it a much deeper pool of manpower from which to draw. It also has considerable resources in the form of coal, iron, and other minerals. Without those resources, Britain could not have industrialized and would not have had the economic might to colonize large portions of the world.

Second, it's similarly obvious why the British developed a larger empire than the Japanese and Italians. In both cases, Britain industrialized much earlier, and the British state was a unified, outward-looking entity much earlier. Italy, for example, was not even a unified entity until the 1860s, and its industrial capacity never came close to Britain's. Japan was a unified entity for much longer, but was effectively isolationist until the middle of the 19th century. Japan might have matched Britain's industrial capacity by the early 20th century, but I'd have to check that. In either case, though, Britain already possessed the world's largest empire by the time those two states began considering imperial expansion. Japan in fact quite deliberately looked to Britain's naval power and imperial reach as a model to emulate.

This leaves us with the French, and why the British empire was ultimately larger than the French. They are comparable in some important ways: France had the manpower to build a significant empire overseas; it had the commercial and later the industrial power to do so as well, and was the dominant power on the European continent through the early modern period. So, let's consider Britain and France in the four areas I mentioned above: the state's fiscal capacity, industrialization, population growth and urbanization, and India.

Now, before diving into explanation, I think it's important to note that the British empire was extraordinarily diverse, and as such there is no one factor that can account for all of it. Rather, we must consider the how different components of the empire were the result of different factors in Britain and elsewhere. So, to divide the empire up, we should start with the Metropole, the Archipelago of the North Atlantic: the islands of Britain, Ireland, and the surrounding seas. From Tudor times, English rulers (both Crown and Parliament, depending on the time) were able to extend their dominance over the rest of the Archipelago. Scotland and Wales essentially joined England as junior partners in the United Kingdom, while Ireland--though nominally part of the UK from the Act of Union in 1800--was always more colony than metropole.

Next, the British empire included a batch of settler colonies. These began with the Atlantic colonies that eventually became the United States, but also include Canada, Australia, New Zealand, and arguably South Africa, or at least parts of it. These colonies were distinct in that large numbers of Britons and other Europeans went there as families, settled in cities or country, and attempted to essentially re-create a British-style society in a new place. They adopted largely British laws and political structures (at least for white settlers), had access to British capital, worked the land in largely the same way that they would have in Britain, and used essentially the same technologies.

There was also a large part of the empire best characterized as colonies of conquest or commerce. For more "conquest-ish" colonies, India and the greater South Asian realm is by far the best example, but this also includes British Malaya and large portions of Africa. In these colonies, Britain not only used military force to subdue indigenous political structures (as they did in settler colonies), but also then governed large indigenous populations either through the imposition of new political structures like the Raj, or through forms of indirect rule. Colonies more on the "commerce" of the commerce-conquest axis include outposts whose primary purpose was to facilitate trade in some way. These include places like Hong Kong, arguably the Cape Colony, and the various coaling and naval service stations around the world like the Falkland Islands or Gibraltar. This category also, for the purposes of this answer, includes the slave colonies of the Atlantic, like Barbados and Jamaica. It's true that Barbados and Jamaica and other plantation colonies were "conquered" either from indigenous inhabitants or from other Europeans, but with plantation colonies the main workforce was typically African slaves. In New World colonies, the Native American populations were heavily depleted by diseases, and thus Britons did not end up ruling over large indigenous populations. The overall purpose of these colonies was to produce a specific product, like sugar, and to this end they imported large numbers of African slaves. This produces quite different colonized societies than instances of direct rule or the conquest of an indigenous population that remains.

How much do you think the growth of British colonies was encouraged by the ability of settlers to own their land (albeit taken from the natives).