Not sure it's down to the size of the country. More likely, it's the size of the community speaking the language, and the closeness of the language to English, together with some other factors.
During my time in Belgium, I found that few Liégeois spoke English, and one simply couldn't get by in Liège without being able to speak French. However, during visits to Antwerp, I could speak in English and people there were happy to reply in English.
So why the difference?
Dutch is a lot closer to English than French. Both are Germanic languages. I know English and French share a large common vocabulary, but grammar and basic vocabulary are quite different.
Dutch isn't spoken much outside of the Netherlands and Flanders, so there's more incentive for the Dutch and Flemish to learn a second language, and the obvious one is English.
Many people in Liège actively dislike speaking languages other than French.
Not many tourists go to Liège.
Brussels, though mostly French speaking, is more international, and gets more tourists, so English is spoken there, though French is preferred.