Not major problem - after all, the animal companion is just a companion and not a major character in itself. Of course, you woul have to realize that every level above 6th the ranger would be slightly more out of balance on the stronger side. But I don't know that it would be a horrible out of balance situation.
If you wanted to provide a more in-balance ranger, what about stripping the ranger of their spellcasting (something many rangers don't use all that much anyway). Thus, a ranger could choose between a stronger animal companion or spellcasting. The in-game justification could be that the ranger has enough time to devote to training his animal companion more rigorously than normal -OR- magic, but not enough time for both.
Just a few thoughts.