Rear rack vs terrapin

Wondered what give you more capacity WITHOUT attaching pannier bags? I am talking just the rack alone vs a terrapin style set up. I currently strap dry bags to my seat post and seat rails. Seems like i can get more on my wife's 26" bike which has an old Blackburn rack on it but on my wolverine I have less seat tube exposed. One benefit of a terrapin system is I can use it on 2 different bike. My mtb does not have braze ons for a rack. I typically have 2 dry bags strapped together under my seat. One has my tent (without poles) and the other has clothes and stuff liked that. I am not sure if my typical cargo can fit in a terrapin bag.