Its the law of deminishing returns.

Each caliber has a "sweet spot" regarding the barrel length to max velocity ratio. After that sweet spot is passed in length, you are mearly adding friction caused by the bullet contacting the barrel, in turn slowing the bullet. Moreover, shorter barrels are typically more accurate. Think about a section of 1/2" PVC 10' long as compared to one half that....the shorter one is more rigid...less oscillation during firing. But, you don't want to get too short...or you start sacrificing speed again....its all about balance.

For a 7-08 (great choice by the way), I wouldn't want anything longer than a 24", but a 20" or a 22" is more like it.