i dont think that a african butterfly fish would do well in a 10 gallon, they are active fish and need atleast 20gallons minimum for 1 i have 2 in one of my tanks
a 20g is twice as big as a 10g P) but that's only in terms of volume. so if you compare the difference in dimensions, the change is relatively small and so the 20g won't necessarily look that much larger.
however, since weight of a tank is driven by the volume, you can't consider the two sizes to be equivalent in terms of tank location. in plain English, this means that a 10g is ok to keep on a table or dresser, but a 20g should really be kept on a stand or other reinforced surface.