You know Dick, even a sailplane can do aerobatics…
What you consider a "sport model" must not weight at least 37lb, it doesn't harm at all if you can do it much lighter, of course.
My figures above are what I think being "typical" for those categories, it is not the same as to say it must be so.
So, I don't think you did anything wrong if your aerobatic model is lighter than my typical "sport model".