I don't think T/C Maxi-Balls are pure lead. I've shot quite a few T/C Maxi-Balls and some that a friend cast. The T/C's will shoot through a deer without expanding IME but a lead Maxi-Ball will expand as you pointed out. I never liked them at first for that reason and I think that's why T/C came out with the Maxi-Hunter which has a hollow point and is probably pure lead. I've got about 10 boxes of each and as you can see the Maxi-Hunters are recommended for deer sized game.
Maxi-Balls are intended for bigger game like elk. I used them on my last muzzy hunt and took a 5x5. The first shot was quartering towards me, bullet went through the shoulder and lungs and ended up under the hide at the back of the offside ribcage. We found him up against a tree about 60 yards away on a steep slope. I shot him again when he got up to keep him from sliding downhill. That shot went in the back of the ribs through the lungs and offside shoulder and the bullet was found under the hide there. I don't think a pure lead bullet would do that. My load was 110 gr of 777 with a fiber wad under the Maxi-Ball.
My Firehawk is probably stronger than the current inlines since it doesn't have a breach plug.