The ender dragon when killed drops only 20-25 levels. While in PC it drops 70+ levels
EDIT by [MCPE Helper] Zeb: To clarify, the Ender Dragon is supposed to drop less xp after the first time you fight it. To quote the wiki:
"Once defeated, the Ender dragon will appear to have beams of light spontaneously erupting from its body. It will then explode, dropping enough experience to bring a player from no experience to level 78 (12000 - 10 drops of 1000 experience, one drop of 2000 experience). Subsequent ender dragons (ones re-spawned via the Ender crystals) only drop 500 experience."
However, in MCPE, the Ender Dragon (when defeated for the first time) drops only enough XP to increase your levels to 68 from 0. (About 10 levels less than in the Java edition.) When defeated every time afterwards, it drops enough to get you to about 18 and a half levels from 0.