According to that article he was 190+ lbs with 9% body fat. That would mean he wouldn't really get below 185 lbs without losing muscle, which is Kovalev's weight in the ring more or less.
Look at his weights in his last few light heavyweight fights. Why would he go down to 169, when he had 175 to play with, if it hurt his performance in any way?
But clearly Spinks was better as a fighter weighing 175. He bulked up to compete as a HW. That is what people have to do. Rocky would be the same. He would not be facing HW as a 189 fighter today.
When Spinks weighed 175lb, he was fighting other men who weighed 175lb. I suspect he bulked up because he didn't want to fight Holmes with a 45lb weight disadvantage.
That's my thinking. I assume he was willing to give up some attributes to reduce the weight deficiency at world class level.
But did he bulk up? I don't think he was ever more than 185 lbs. It could be that he just stopped cutting.
Maybe he fought most of his career at 175 because its much, much easier to fight 175-lb opponents than 200-220lb ones, all things equal? Maybe he went up because he knew that he could do so, given his frame, without giving up any attributes?
I think Foster got to 192...maybe for the Frazier or Ali fights. Plus he had a few "heavyweight" fights back in the 1960's. (Since Boxrec has changed its format....harder to find weights of fighters for particular fights)
Because Larry was 40-50 pounds heavier. Mackie says Spinks' walking around weight was 187-192. He just changed his percentage of body fat. Why would he have dropped to his normal weight of 175 to fight Holmes?