My favorite part of this sentence is the exclamation point. Why anybody would ever need to shout about their shoulder size is beyond me. At this point I am only using Duolingo for these kinds of sentences.
Would "my shoulders are big" be more natural in English though? haha
Both seem equally natural to me, and that would be Mabega yangu ni makubwa in Swahili.
I've got big shoulders means the same as I have big shoulders, and it sounds more natural in British English