Does a creature with Lifelink provide lifegain equal to its power, or the defender's toughness?
Yes, you've been shortchanging yourself!


  702.14b Damage dealt by a source with lifelink causes that source's controller, or its owner if it has no controller, to gain that much
  life (in addition to any other results that damage causes).


A creature with 10 power blocked by a creature with 1 toughness deals 10 damage to that poor little blocker.  Creatures with 10 power are not known for their discretion or moderation; they don't deal 1 damage on the grounds that that will be enough to do the job.  No, they smash that blocker into smithereens and leave behind a smoking crater!

I think your confusion may arise from the old card Drain Life, whose text reads:


  Drain Life deals X damage to target creature or player. You gain life
  equal to the damage dealt, but not more life than the player's life
  total before Drain Life dealt damage or the creature's toughness.


Drain Life is the exception though, not the rule; for lifelinkers the creature's toughness is irrelevant in determining how much life is gained.  (See also though Abattoir Ghoul, a recent card which offers a kind of new spin on the Drain Life idea.)