Its hard to single out specific muscles, but just understand that frequent round back pulling will reinforce that technique and exacerbate any weaknesses/imbalances that lead to that being your strongest pull. On the other hand, flat back, good tech pulling will reinforce better form and work on the muscles that are weak. Work on bringing our flat back technique up, and your overall pulling strength will improve and form should get better. Your absolute true 1RM (regardless of form) might still incur some rounding but I'd avoid making this the majority of your training unless your trying to be a specialist like KK. Which you probably shouldn't unless your on his level.
Chad Wesley Smith really nails it here: