There are four main things that affect the catch rate of a Pokemon:
1) Which Pokemon it is (a Legendary will always be hard to capture, regardless of what you've done to it).
2) Their current HP (why you lower a Pokemon's HP before catching).
3) Any status ailments (e.g. Sleep).
4) The one that answers your question: their total HP. The other three factors alone would mean a level 100 Patrat would be as easy to catch as a level 1 Patrat. However a level 100 Patrat would have far more total HP than a level 1 Patrat, and therefore would be harder to catch. How much of a difference this makes, I don't really know.