The fact that it's solid wood doesn't mean that it's any good or that it will survive the rigours of being played for a significant length of time.

It's largely a matter of balance. A heavily built instrument will last for years but sound awful. A lightly built one will sound good but fall apart. An expert builder will know how light she/he can go whilst producing something which will last. It's not simply a case of sticking a flat piece of solid wood on top of a box - the top needs to be "tuned" to sound its best (and most of the better mandolins have carved tops anyway).

As with guitars, at the cheaper end of the market, you're sometimes better off getting something built of plywood (ah! My old, red label, Yamaha FG180 and how about spending a couple of hundred thousand on an an original "plywood" Maccaferri?)