The point is, IMHO you need to qualify for the challenge.
In my game, in a straightforward challenge of strength, let's say an armwrestling match, STR 17 guy beats a STR 16 guy almost always, simply because he's stronger (although hangover might affect STR temporarily). You just don't go challenge the biggest guy in the village and hope for a lucky dice roll. If both were STR 17, then it's a challenge. Until that it's back to the gym for the lad.
Of course when more technique comes into play, like knocking the other guy out of balance, Athletics roll would come into play with appropriate modifiers like the one Mugen suggests (like STR difference * 10%).
For me, basic lifting falls into the straightforward strength challenges category and only requires roll if it's around you maximum capacity. Of course, this is where good GMing comes into play, so obviously the door leading to salvation has equal STR than your strongest player. If not, what's the point of it being there. It's a waste of time rating SIZ's for the objects in the game and then spend again time calculating chances of success, if they're meant to be opened anyway. Either it can or it can't. Consecutive attempts shouldn't make any difference, nor letting a weaker guy try the task.
If a door can be opened, either it's coupled with some tension (bad guys about to storm in) and calls for a roll, or it eventually opens after some trying anyway.
Same goes for lots of other tasks.. only the SIZ 17 guy rolls whether he'll get stuck in a tunnel. Smaller guys should get through and the bigger guy realizes he won't fit. Again, if GM decides that this part of the scenario could add a twist in the game, he sets the limit to something that calls for a roll for someone.
This sort of approach in general makes the game more logical, realistic and smoother. You can deem that the Tower is so difficult to climb that only your most nimble PC (decided by DEX) has a chance of trying.
This way you avoid the common situation where every PC asks for his turn to have a go. If the task was quite easy then have higher DEX PCs succeed automatically and only call for a roll for the clumsiest climber.