This seemingly casual joke outright confirms that they cannot fully control their forms, regardless of how good they are with their foxjutsu. This explains a lot why in previous chaps, Kogane could not just change her weight.
This is purely my opinion (I'm only ever one chapter ahead of you guys, it keeps me motivated), but holding human form seems to be so onerous, decades of training, that I think it's something like doing ballet, playing piano, driving a car... At first you're thinking about every little thing, but then as you get better you just do certain things without thinking about them. A concert pianist is definitely not going, oh which finger should I put over here and what muscles do I need to use?
So I bet that over their decades of training they settle on their ideal human shape because that's the least effort for them to hold. At that point they're not thinking about it much other than tail & ears or not? So if you wanted larger breasts, you're looking at years of constant retraining to get them to the point you can hold them without making a focused continuous effort again.. Like learning to walk again after a stroke. That's why Mugi is so concerned about losing the transformation last chapter, Mugi's not quite full autopilot yet.
I bet there are a very few geniuses who don't find it too hard to do differing appearances.
Then there's the final bit, which are slow concordances between the real fox form and the human form. The human forms seem to age as the foxes age, which maybe reflects their core mentality (so it affects the transformation too). And as you mentioned, the weight. What I think (again pure theory) is that some portion of the food calories go to the fox body, and then the human form gets fatter because the real body is fatter, like the aging.
TL;Dr the transformation is mostly on autopilot after a few decades, very hard to consciously modify. Which is what you said with a million more words.