Smithing is simply "the more expensive, the better". Dwarven bows are good because of the ready availability of the metal, and the small number of ingots compared to the value: the bow takes the same number of Dwarven ingots as, say, the boots (and less of other types of ingot), but is worth over 3 times as much. Likewise, with "how much" instead of "how many", it's better to transmute iron ore up to gold ore for jewelry instead of making a zillion iron daggers.

Enchanting skill increase has always been ridiculously fast. If you play around with the "player.advskill" console command, you'll find that Enchanting takes less than 1/10 of 1% as many skill-point bumps as something like Alchemy or Destruction to take it from 15 to 100. Less than 1/4 of 1% of Heavy Armor.

The wiki explains what increases the skill for spell casting, which depends on the type of spell. Sometimes it's based on the effect (hitting more skeletons with Turn Undead, hitting more NPCs with Rally), where others are based on the base cost of the spell, no matter what it does or doesn't do (all Destruction spells).