You are absolutely right, I have seen jumbo tosai that aren't any good. They are used as a device, sometimes, just to make more money.
A 7" tosai is a jumbo tosai if it comes from jumbo parents. Jumbo Tosai should really only be considered Jumbo if they have the ability to grow to a very large size. If they come from jumbo parents they are more likely to achieve that growth and should be considered jumbo tosai no matter what size they start out. If a dealer is selling jumbo tosai and the parents are only 24" then they are not really jumbo tosai. The term "Jumbo" does not refer to the size of the tosai, it refers to the parents it comes from. I do not consider 80cm Jumbo.
Quality in any size tosai is undeniable and recognizable.