Quads have a bit of "the rocket equation" problem to them. The more mAh in a battery the heavier it is so the more thrust you need to lift said battery so some of the power you just added is being used to lift the source of the power into the air and at some point you are adding more weight but not able to create the thrust need to fly and at to efficiently run things, you're just losing flight time by adding the weight.
Basically a motor is just a bunch of wires and magnets, if you increase the current in the wire you increase the magnetic force or torque but at some point a motor becomes "saturated" you can't push more current through it and it's just burning off the power being pushed through it in the form of heat (vibrating the atoms of air around it) and electro-magnetic radiation, as heat goes up so does resistance and so things either level off or you burn the enamel coating off the wires and a short happens and then the wires themselves or other components tend to get roasted. Running motors at high RPM also puts the fluid in the ball bearings under "stress" from heat build up so eventually blow out the bearings holding the shaft in the motors but point being there's just some upper limits of how much power can push through a given motor before hit it's max RPM or start cooking components.
If somehow we could add more power onto the quads without adding more weight then this would be a non-issue but basically if adding more weight then need more thrust to overcome gravity, if need more thrust then need to either drive the motors at higher RPM (fighting wind drag on the prop blades) or need bigger surface of prop blade, again more weight of prop and more drag means harder to spin at high RPM if is bigger though so is trade off there too. Feel like 9/10 posts I have here I link to this site but ecalc.ch good site for running numbers even with free options to see how adding more mAh or changing prop size on a given quad will effect the thrust and flight time numbers. Basically a quad with 1:1 thrust to weight doesn't get off the ground, it needs more thrust force than downforce from gravity to get in the air, but to stay stable it needs even more power because the thrust vector it creates isn't pointing straight down as soon as it tilts a bit from wind or any other external forces, basically the higher thrust means more control for the end user because the quad isn't spending all it's power just staying in the air and level.