One of the interesting and weird things I saw in One Piece was the mermen and mermaids in the Fishman Island Arc. But one of the biggest confusion I have is the relation between Shirahoshi and Otohime. I don't know how Shirahoshi can be a real daughter of Otohime. Otohime having such a small body and Shirahoshi being huge even in baby stage. Anyone has any ideas?
I found this in SBS Vol. 10
D: When the Arlong guys and the other Fishmen give birth, do they lay eggs like fish?
O: BZZZT. Fishmen are still mammals, so they give birth the way humans do. There are fishwomen too, of course.
From this we can confirm that Shirahoshi was not born from egg. Fishmen and Fishwomen are mammals.
If Shirahoshi was not born from egg the most logical explanation behind her birth is that she was probably born small and had a massive growth spurt. Growing to a massive growth is not impossible because she has royal blood and her father Neptune is of a giant size merman after all.
So the answer to your question is yes. Otohime is the mother of Shirahoshi even with differences in their size.
I don`t remember where I saw this, probably in some SBS or databook, but there are two separate species of fish-human hybrids. One of them is the fishmen/fishwomen, which are the ones that walk on humanoid legs like Arlong. The other species is the merpeople (mermaids and mermen), wich are the ones who need bubbles to move when not in the water, such as Caimie and Shirahoshi.
So Oda said FISHMEN are mammals, but he didn't say anything about merpeople. Meaning Shirahoshi could have been born from an egg, and some real-life fish even fertilize eggs externally so maybe Neptune didn't even need to copulate with Otohime.