Water cut is done by percentage of body weight. Generally speaking most UFC fighters that are serious about cutting do around 10%. Some fighters, like McGregor, can go as high as 15%, which btw is dangerous. At 170, if McGregor cut 10% it would put him at 153, so he's probably cutting 13%, maybe a little more to get to 145. Its a huge cut and why he looks concentration camp at 145. I doubt RDA cuts down water from more than about 180 pounds. A 10% cut from 180 puts RDA at 162, so same thing, 180 down to 155 is about 13% cut. These are huge cut numbers for both fighters. At 15%+ you can actually lose pressure in your veins, and they can collapse and you can die. Or have kidney failure, etc.
I would guess when they enter the cage RDA is no more than 180-185 max, and McGregor might have had enough time to get his number to 175. RDA should have the size advantage since McGregor has been cutting to 145. Not enough time to put on all the muscle.
