I'm aware that the maximum wattage allowable for a hi-hat is dictated by the trim and type of lamp.

Because I don't like the visibility of the socket and bracket on a standard Halo open type and even the stepped baffle trim, I install cone-type trims that completely obscure the lamp socket.
The label permits R30, 75-watt lamps.
The builder wants me to install R40 lamps in order to "fill-in" the trim more.

Although I refuse to do so, I am wondering what the issue may be if the wattage is the same, but the diameter increases?

Closer proximity to the trim?
Less heat dissipation?