'~을걸?' means 'maybe~' or 'perhaps~' and other expressions like that. as your example, 많이 다르다 >> It's much different. 많이 다르지 않을걸? >> Maybe It's not that much different. I'm learning English so my answer may or may not be correct grammatically. but I hope you get it.

Hi. :) We usually say '을껄?' to express a possibility or assumption with less conviction. It think it can be translated like 'it might be ...'. Thus, "많이 다르지 않을껄? (or 많이 다르지 않을 것이다)" sounds like "It might not be that different". (By the way, is this sentence right??) For example, 1) A: 네 동생은 뭐하고 있어? (What is your brother doing now?) B: 밥먹고 있을껄? (He might be eating a meal now.) 2) A: 여기서 병원까지 멀어? (Is it far from here to a hospital?) B: 많이(or 꽤) 멀껄? (It might be far from here.) Hope it helps you.