Bally heck - no not really, it is to do with the particular runway / obstactle weight limits, and most of the time the 738 and the 757 will derate its power so that it is at the runway / obs / tyre limit for each particular case.
What you say is correct if they are at full power.
Many of the 757's used as described in the UK have artificially reduced MTOM's in order to reduce overflight charges.
Perhaps it is complicated by whether or not other performance includes an increased V2 for better climb etc.
As the 757 (and I guess the 738) have less flexibility over climb thrust derate, perhaps the 738, being less over powered uses increased v2 procedure for obstacle clearance more, meaning it stays on the rwy longer ????