DSLR's have longer battery life unless you're constantly taking pictures (and consequently are lighter since the battery can be smaller), since the sensors is typically off (whereas in mirrorless it's constantly going to the EVF).
The majority of the size of the camera is going to be in the lens anyway. Mirrorless is not going to make your telephoto any smaller·
The two advantages mirrorless really has over DSLR (other than... lack of a mirror which makes it cheaper) is faster photo rates (since you don't have to move the sensor, though modern DSLRs have liveview anyway, though not through the VF), and the fact that the EVF shows the effects of ISO/exposure/etc (though again, modern DSLRs have liveview, though not through the VF).
The majority of the size of the camera is going to be in the lens anyway. Mirrorless is not going to make your telephoto any smaller·
The two advantages mirrorless really has over DSLR (other than... lack of a mirror which makes it cheaper) is faster photo rates (since you don't have to move the sensor, though modern DSLRs have liveview anyway, though not through the VF), and the fact that the EVF shows the effects of ISO/exposure/etc (though again, modern DSLRs have liveview, though not through the VF).