First boost is 1-7, second is 8-12, then from there it's 13-19, 20-34, 35-54, 55-84, 85-124, 125-199, 200-249, 250-299, 300-349, 350-399, 400-449, 450-499, 500-549, 550-599, 600-649, 650-699, then finally 700. Anything over 700 is wasteful as damage and defence won't increase any further.
In between 1 and 7 you will do the same amount of possible damage no matter what. So a pet with strength and defence of 1 will do the same as a pet with strength and defence of 7, but once it hits 8 it'll do more damage. Speed/movement does nothing anyway so you shouldn't train in it, and HP is health... so doesn't get a boost. Same with level, although at level 50 you get an extra type of attack called berserk which is really quite handy.
Ahh, see, I didn't know that.
Learn something new every day. <3