Pistol-mounted lights require specialized pistol holsters. One of the reasons I like the Lightguard is that, in addition to the grip-activated membrane switch that doesn't require any addition motions to activate, it adds hardly any bulk to the gun and therefore hardly any bulk to the holster.
The TLR-1s is obviously much brighter, but requires a separate motion to activate and is also larger, necessitating a fairly sizable holster. Some people carry those things IWB, but for me, it pretty much relegates the gun to OWB carry or nightstand duty.
Another issue is that the big lights require the holster to be fairly generously cut to get the thing in and out of there, like the RCS Phantom in the picture below:
here, I can't actually get a finger in there in front of the trigger to pull it, but depending on the gun/holster/light combo, it seems like a possibility of which it would pay to be aware.