A most interesting query indeed, but I should like to note that before reading my take on this question, there is no actual answer as far as historical sources go, we have no records of the conversations which occurred between King Albert I (the Belgian representative at the Paris Peace Conference) and the rest of the plenipotentiaries there (at least regarding the question of territorial concessions. With that preamble aside, let us begin by approaching this topic in two "concepts of Versailles" as I shall call them: self-determination and territorial integrity.

Before even that however, if you shall excuse my pedantic side I would like to note that Belgium did not even "get" Eupen-Malmedy as you might believe it did. Here's the actual text from the Treaty of Versailles on that territorial change:

Article 34: Germany renounces in favour of Belgium all rights arid title over the territory comprising the whole of the Kreise [Districts] of Eupen and of Malmedy. During the six months after the coming into force of this Treaty, registers will be opened by the Belgian authority at Eupen and Malmedy in which the inhabitants of the above territory will be entitled to record in writing a desire to see the whole or part of it remain under German sovereignty. The results of this public expression of opinion will be communicated by the Belgian Government to the League of Nations, and Belgium undertakes to accept the decision of the League.^(1)

So even with this minor territorial concession, the "Big Three" powers at Versailles were reluctant to even properly allow Belgian control over the two towns without the express approval of their citizens. From the 10th of January to the 23rd of July 1920, these registers were laid out in the town halls of the two cities, and I'll allow this quote from a publication on the matter to display the result:

When the registers finally closed on 23 July, only 271 names out of an eligible total of more than 33,726 inhabitants appeared on the register of protest.^(2)

So, what exactly prompted Clemenceau, Wilson, and Llyod George to even bother dealing with these minor territorial concessions? Therein lies a rather tricky answer, but let's begin with the concept of Self-Determination. The seventh of Woodrow Wilson's famous (or infamous) "Fourteen Points", it simply calls for the following:

VII. Belgium, the whole world will agree, must be evacuated [removed from danger] and restored, without any attempt to limit the sovereignty which she enjoys in common with all other free nations. No other single act will serve as this will serve to restore confidence among the nations in the laws which they have themselves set and determined for the government of their relations with one another. Without this healing act the whole structure and validity of international law is forever impaired.^(3)

Note that Wilson did not mention a "compensation" for Belgium on the part of the ravages Germany had wreaked upon her during the war. This key wording made all the difference to his stance (and by extension that of Clemenceau and Lloyd George) during the actual conference itself. However, Self-Determination also called for nations to rule themselves, so a large part of the reason why ONLY Eupen and Malmedy was given over to the Belgians was purely a matter of ethnic makeup: around 10,000 Walloon citizens resided in Malmedy and had done so for a century under the rule of the German Empire.^(4) This simple consideration meant that conceding this territory to the Belgians complied perfectly with the notion of self-governance (ignoring for a brief moment the fact that both districts were also relatively agricultural in their economic importance).

Now let's address the argument you've posed in your second question: why not more? It seems entirely logical that the nation which suffered so much during the Great War with a direct occupation for almost four years receive some territories from the nation which had made it suffer so much. But this thought entirely ignores a slightly thorny issue: German continental territory was never invaded during World War I. At the time of the November 1918 Armistice, not a single square kilometer of Imperial German land had been invaded by any of the Entente powers.

This posed a massive problem when it came to Belgian demands for territory: how could they reasonably demand something that wasn't rightfully theirs? After all, the towns and kreise listed above (Bon, Aachen, Cologne and so forth) were predominantly German in ethnicity and identity, claiming that they somehow belonged to Belgium would not only infuriate the German delegation at Paris, but also give the Big Three a rather amusing spectacle. One of the key reasons why the Rhineland was administered by the League of Nations (though militarily occupied during different phases by Entente troops) was because of these considerations as well.

So there we have it then. A Low Country which entered Versailles with the hopes of not only restructuring its territory but also gaining new ones from the enemy which had taken so much from it during the war. A Low Country which in the end, only received two minor districts which were hardly economically vital or massive in population. a Low Country which had essentially been 'betrayed' by the self-governing principles of Wilson and the complexities of getting a country that had not been properly invaded to give away more territory.

Sources:

[1]: Treaty of Versailles from the Library of Congress

[2]: O'Connell, Vincent (2013). ""Left to Their Own Devices". Belgium's Ambiguous Assimilation of Eupen-Malmedy (1919-1940)" . Journal of Belgian History. 43 (4): 10–45.

[3] Transcript of Woodrow Wilson's January 8, 1918 speech to the US Congress presenting his 14 points, from the US Embassy and Consulate in the Republic of Korea.

[4] O'Connell, "Left to Their Own Devices", 11.