Reading something that Rox said: He says that only Harald can take full advantage of the 1/2 fruits he wants. Meaning someone else can eat it, but it won't be the same.
Momo is a counter to this, but maybe base size
does affect Zooan size for this specific fruit?
I was actually speculating that the fruit was a Giant Giant fruit, and that if a giant eats it, it will unlock its "full potential", since for a normal human it'd just turn them into a regular giant. But I forgot Sanjuan Wolf basically did this already.
But maybe the logic was correct? The reason why only Harald can unlock its "true potential" is that an Ancient Giant can make the Zooan form much bigger than anyone else?