Nah, you've fallen in to a classic trap for men. Even though the answer could be correct under different framing, it's not always okay. The framing matters.

So if she said would you still love me if i was the size of a whale

In this example, the underlying insecurity is about you. She's worried you'll leave her if she's not always at her best. Thus just saying yes provides helps solve the core issue. To be honest it's not a perfect answer, but it's fine.

gf saying "I'm fat"

In this one, the insecurity is not (just) about you. Most likely she's worried about how other people perceive her, or how she perceives herself. Men often assume any concern someone puts into their appearance is for their partner or for finding one, but it's not. Saying you'll love her even if she is fat does not address the underlying insecurity. In fact, it implies she is fat and heightens what she is worried about.

I would advise a hug or something for immediate reassurance and then asking her some gentle questions to gauge what she's really worried about if you're not sure. Literally, "hey what brought this on?". Maybe with a "you look great" leading into it first.

Although when she asks you would you still love her if she was a worm, the correct answer is yes.

Better to put it back reframed in more direct terms, showing you understand the underlying insecurity, but dodging having to be dishonest about the fact her being a worm obviously would change things. Then lighten the mood with a joke.