Because tank there is problem to fit it in the content. If all items are same size you dont have to adjust their size when they are close to each other. Check out the forge source. Almost all images has 50/55/65/70px set to fit.
I already updated monster items, gem bonus items, DPS bonus items. On my PC I almost finished to cut all other items.
Not minimal/maximal. Exact 90x80.
I almost finished to cut them all. I will send you file with all items. Each on separate layer.