In Eagle, why is the tstop layer larger than the pad itself?
The TStop layer is for solder mask, the thin coating that prevents copper on the board from being exposed except for the areas where desired.

The TStop layer leaves apertures slightly larger than the pads so that the copper is still exposed in case of misalignment when producing the PCB.

However, extreme misalignment can cause copper to be exposed by the wrong aperture and you wind up with a situation where two nets are both easily shorted by solder in the same aperture.



From Printed Circuit Design &amp; Fab

Some PCB software will let you change the amount of pullback of the TStop layer, but ideally you should have some so that minor misalignment doesn't hide some of the pad, especially with fine pitch pads.