Translation:I have big shoulders!
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 think it is common in English to speak of "broad shoulders" rather than of "big shoulders". I don't know if that would require the Swahili to be "mabega mapana" or if "big shoulders" could also be translated as "mabega mapana."