The problem with var is it may or may not put weight on you. You don't know until you invest in it, and run it. Chances are it's not going to put a bunch of muscle on you - law of averages.
Var doesn't increase strength more than many other AAS, Var's ability is to be able to increase strength with little to no weight gain which make it attractive to many.
Var also does burn fat. It most certainly does, especially in already lean person, because it burns viceral fat. I can attest to this. Even while on a high calorie diet, my waist shrinks with NO cardio. It's also research proven. That said, it's not a magic pill and does not seem to cut me anywhere else without me putting in the effort - which is why people who are not already lean will not see immediate and easy cutting results from it.
It's definitely not a mass drug. Adding it to a mass cycle will not increase the lbs you put on. It's almost best as a recomper - slowly replacing fat with muscle, which is why people don't necessarily change/gain weight on it, but do put on lean muscle. And I love how var make me look - var and primo make me look so damn good, haha.
But anytime someone has a goal to gain 10-15lbs, I'm not going to recommend var. They may be the lucky few to gain that much, but who knows, and it's an expensive endeavor just to see. If major gains are wanted, then use a compound that will 99% of the time get the job done - baring excellent diet and training, etc.
And var ain't side free either - it has it's hidden cholesterol sides which are nasty for me (0 HDL, hello), and it can cause constipation, gastric upset, difficulty eating...But it is mild on the liver, if you get the real thing, because it metabolizes mostly in the Kidneys. And there is another worry with var as well - getting the real thing. I'd be inclined to say many of the success stories about gaining on var are due to the possibility of it being cut with something else, or something else all together.
Anyway, off my soap box.
And to address your question about if it makes you stronger, why doesn't it by default make you bigger. Var's main actions are directly at the receptor level, and mostly anabolic, so it's a good tissue repairer. But it doesn't do much else because it lacks any real angrogen properties, that's what makes it so good for muscle wasting. I also believe IIRC that Var's strength increases come from a different pathy than other AAS's CNS augmentation, but rather from the increases storage of creatine in the muscles.