Kinda makes sense to me.

It seems he has different modes.

At his 25%, he increases to 1.5 times his normal size, and gains the strength and durability appropriate to a "normal" man his size.

At 50%, he grows even larger, while also gaining strength and durability for a "normal" man that size.

At 75%, he is at the size of his 25%, but more power is built up within his body, granting him more power than his 50%.

At 100%, he's normal size, but all his power is concentrated on increasing his strength, stamina, and durability.

Of course, all that built-up power is like a carbonated soda in an unopened shaken-up soda can. It has enormous strain on his body, so he needs to rest and recover after use.

I don't see what's so confusing about that.