Some people learn fast, other take a bit more time. If you really are a quick study as it seems you are, I'd say you'd be OK with a 60 sized aerobatic plane. The instructors probably errored on the side of caution, which is what they should do. They wouldn't be very good instructors if they allowed you to bite off more than you can chew.

However, I'd caution you that a aerobatic plane can be a handful compared to a trainer. I've seen many a pilot get good with a trainer, then they jump to a low wing semi-scale aerobatic only to drill it in within seconds of takeoff. I too was a quick learner (soloed my first flight), but I dang near lost my first low wing aerobatic on takeoff. First turn, that sucker rolled so fast I went inverted... talk about pucker factor. Take Crash Guy's advice, get help, a spotter, buddy box, whatever.

Cheers.