Bra sizes after D cup are a pain in the caboose ...
"There are enormous differences in cup size notation between manufacturers. This is especially true in sizes above D in the US, UK, AUS and NZ. In general, sizes between AA and DD are fairly standard but once we get above this there is little or no coherence. Some manufacturers use size DD but then continue with E, F, G, H, I, J, etc. Others use DDD to represent an E and some go on to use E, EE, F, FF, G, GG, H, HH, etc. This can make purchasing a large-cup bra difficult."
Ps - the difference is bigness / fullness haha