well, denmark and belarus really only have one player each (okay 2 in the case of BLR), so it's not that strange. the czech republic on the other hand has 8 players in the top 100 (i include no. 101 hlavackova for the sake of it ), and that's really impressive.
Funny that you started thinking about it just now I mean, Switzerland have both Federer and Hingis, Belgium at the same time had both Clijsters and Henin Serbia is a small country, too.
Btw, I think Imperfect Angel is right