In the prepubescent ages the male and females body frame is not that different. But after that it starts to change considerably (for the most part) If you adjust the chest part to be more diminutive I'm sure the male shirt would fit your Sim better.
I play with a young adult sim and the chest size isn't something I would want to change honestly
Then I suggest that you give your Sim clothes that fit her better.
There simply is no simple "fix". As I explained to the OP there are differences in the male and female bodies that has to be taken into account. They changed it so that males could use female clothing and the other way around knowing that much of it would not look very good on the other gender just as in real life. Both you and the OP are welcome to suggest that they make male/female conversions but there simply is not a simple "fix" to the differences in the body frames of the sexes. There were two things the OP and you brought up. Male shirt not fitting very well. The male torso simply is much bigger than females. You mention that female jeans don't fit males very well. This is because females have wider hips than men.
I hope this has helped you understand what is involved.
