Welterweight is 156lbs - 170lbs.
Non title fight allows a one pound so 156lbs to 171bs here.
There are a few examples of guys being extremely underweight for the division, eg BJ Penn
weighed in at 167.2 lbs for his fight with Fitch, and that was wearing jeans and a belt. He was likely about 163lbs or so.
Dan Henderson weighed in at 199lbs for his fight with Cormier, on video eating full meals the night before and day of. Probably a solid 25lbs+ weight difference between those guys and it likely should not have been sanctioned in the first place.
Conor is a solidly sized lightweight that makes for a small welterweight. Same goes for Donald. I don't think there is much of a size difference here.