There's a large amount of crossover between growth and expansion, to the point the two have become mostly synonymous.
TLDR of the definitions: *_expansion tags were meant for existing body parts to increase in size, while *_growth was meant for growing new body parts, usually via transformation (e.g. male characters developing breasts, or female characters developing a penis)

The phrasing of the tags is the main culprit here.
Most people will think 'their breasts are growing' rather than 'their breasts are expanding', so they use 'growth' instead.
Search: breast_growth -breast_expansion -gender_transformation (The gender_transformation tag is excluded as most of those are the appropriate use of the tag.)

I think the best course of action for this would be to alias the *_expansion tags away to their respective *_growth tags, then change the 'growing new body parts' suffix to something like *_development for clarification.
I can make a BUR for this, but wanted to field response and ideas before putting in the work.

Edit: development is currently being used by 4 posts as a mixed definition. Two posts are game-dev progress pics, one is expansion, and one is character design progress.
May be best to disambiguate this to tags like game_development, anatomical_development, and character_development or something like those

dirtyderg said:
I think the best course of action for this would be to alias the *_expansion tags away to their respective *_growth tags, then change the 'growing new body parts' suffix to something like *_development for clarification.
I can make a BUR for this, but wanted to field response and ideas before putting in the work.

I think sprouting works better for growing something new. Development can have the same problem as growth, as it indicates some kind of maturing, including growth/expansion of existing anatomy (e.g. a short young girl with small breasts developing into a tall adult woman with large breasts). I'd actually say "development" has stronger connotations to something pre-existing that changes, rather than something new coming into existence. "Sprouting" I think works better to indicate something new that wasn't there before.

Seems you've sprouted a new thread