For me it has a few factors.
*Proportions. If a character is able to walk straight with breasts several times the size of her head it seems wrong. Yet if it is shown that the breasts are effected by weight and gravity I am more okay with it.
*Race. Most of the reptiles I prefer to not have mammaries.

That's how i see things like. a bit.

just seeing females with back breaking breasts that don't fit their bodies just don't seem possible, i know it's the fandom but still.

To be honest, It's really nothing wrong with small and normal size boobs.

I can understand it's your opinion, my friend.