I mean, in the genre section in page 1 it clearly says "big tits" and "big ass" xd.
All jokes aside, it is a valid complaint. Looking at other works from the artist, there does seem to be more voluptous women than slender ones. This could be what he is more comfortable drawing, but he also has works of slim girls which turned out quite good if you ask me.
To be honest, I personally prefer more well endowed attributes so I may be a little biased, but I also think this to be the preference of the majority. As you said, this is justified lore wise for the succubus, since these attributes are considered generally more seductive so succubi would prefer to have these features. However, I have faith that in at least every game we will get at least one slender girl, be it a succubus, an angel, a human, or an elf.
General Gloriosa did not have a huge chest like the others, and I think that was a good move from the dev's parts so that he can cater to other people, but it seems you still want even smaller breasts than that :ss. Also remember that lore wise, succubi from the leaf clan develop their bodies the quickest, so maybe some succubi from others clans could be more of what you're looking for.
As for loli characters, the dev has already stated there won't be any, since they could bring trouble for obvious reasons.
All in all, I'm all in favor of the current body proportions that we have, so keep at it :3, but I will not get mad if you decide to sometimes include slimmer characters like General Gloriosa to appeal to other tastes. The art is great either way, great work to you Zion, Kainkout and IND3X.