Why do cats get bigger after getting them fixed?

I know you all are going to say it's a myth but I have 2 cats, sisters from the same litter, I feed them exactly the same, they eat the same amount, but one is fixed and one is not, and the one that is fixed is way bigger than the other one even though she was the tinier one before she got fixed, and the one that is not fixed is the size of a 9 month old kitten and refuses to grow (both are 2 years, 3 months, 1 week, 5 days... obsessed, I know) . And don't yell at me about getting her fixed, she's an indoor cat and she's terrified to go outside, so I don't see it worth the price. Both are indoor but the one that is fixed is a little more adventurous so we got her fixed in case she ever does get out.