The difference usually refers to the height of the arch, length of the legs and shape of the heart and arms, where French is low, short and pointed and Belgian is high, long and rounded. The arch and legs affect the area of wood remaining over the arch, but the rounded heart only accentuates it. As you can see here, the legs and arching of French bridges can be higher and longer as seen here.

http://schuster-stege.de/Stege_2_E.html