I searched 要 in chinese dictionary and finally I got it.
to my surprise, I didn't know this kind of meaning either !!!
Am I chinese??? :D
(8)表示比较;估计。
他年纪稍轻一点,脸也要瘦一些。——巴金《家》
so,
我比你大。---- I am older than u (100%).
我要比你大。---I am older than u (90%).
我要比你大吧。----I am older than u (70%).
我应该比你大吧。----I am older than u(50%).