Hello there, I have a female that has been looking decidedly fat over the last month or so, I think she is gearing up for egg laying! Like you, I feed all my axies the same amount, so this is the only thing I can put it down to.
I have a male in her tank with her now, waiting for the right moment when she's 'in the mood'!
The only other thing to watch out for is bloat, but I believe this shows as a general all over swelling, throat, limbs, everything, rather than a fat tum!
So yes, I reckon she's full of eggs! If you don't want lots of baby axies, it might be a good time to separate them if you think you have both sexes.