Imagination or possible?
Heyy I go to the gym and eat in a deficit and I really mean in a deficit, my weight stays the same the whole time, probably because muscles are heavier than fat, I have no idea but in any case my stomach is thinner but I have the feeling that my legs are getting bigger and bigger, is it just my imagination or is it really possible?
If you do intense leg training, it is already possible. More muscles.
Despite deficit?
It would be possible. Or cycle-related water retention.