My personal thought on this is that DoH can level really quick but everyone does act like you can do levequest with HQ synths all day long. Cause I can't do that. I don't have enough gil to buy those HQ materials and not everything can be gathered either. The question is what SE thinks about the right amount of xp a class should get. And in case of Mining/Botany its definatly too slow. Someone said that leveling DoM/DoW is even slower but that's not true. Compared to miners/botanists you can spam fates, do story quests and leeves all day long.

And another thing is that you shouldn't think in short terms. SE will increase the level cap some day and at this rate it would be horrible to level miner/botanist to something like level 75 and one million xp per level. We're not even close to the maximum level this game is going to have now.