I see a little better what you are getting at. Just FYI, that's not negative only training, but were talking semantics here lol. Pure negative training is taking a weight you can not lift one time, and slowly lowering it, having assist to get it up and repeating.
Your talking about end of set negatives which can be very beneficial. I say go for it, this is all an experiment anyways ha ha. That's what makes it fun.
As far as gaining 'size' while losing or even remaining the same weight, that's a pipe dream perpetuated by the supp company's.
If you gain size, were do you suppose it will come from, thin air? You must have a calorie surplus in order to gain "weight", IE. muscle.
See the problem here? If you don't gain weight, IE muscle, again were will it come from to be on your arms.
When you figure that out please let me know, ha ha.
One caveat. When you are just first starting out, you may gain quickly and even lose some bodyfat. This quickly comes to a raging halt, and frustrates the hell out of new trainers. This is were patience and maturity come in. Keep plugging away and the gains will come.
Just probably not as fast as you would like.
Jesus sorry for the novel.