I think that in order to be able to seriously talk about the Fury - Usyk fight as a competitive one, it is necessary that Usyk gain at least a little weight (can he, given that he has already moved to a new weight category for himself?). Now Usyk weighs 100 kg, and Fury 123 (or already more) - 23 kilograms of difference, in fact, they are in different weight categories.
He will lose his speed and probably his accuracy if he gains more weight than he can afford, he is ok with his current weight as long as Usyk maintains that speed and accuracy, he can fight anybody as he did with Joshua and beat him, heavyweight is the only division where there is no maximum weight limit, Usyk if ever will add more weight it will not be more that will hinder his speed.
I also don't think that Usyk needs to be heavier to beat Fury. I mean if he will ever fight against fury then he probably has to be even faster than in the fight against Joshua in order to be able to dogde and avoid furys fast jabs. I think it would be bad for Usyk to increase his weight further than he absolutely has to in order to keep within the weight limits. In the end being heavier "only" gives you more punching power and i don't think that a little bit more punching power would drastically help Usyk, but the downsides of more weight, the loss of speed are definitely very dangerous for him. If the fight of Fury vs Usyk will ever happen i would bet that the fight goes over the full distance, because both of them are not really heavy hitters.