Some time ago I gave a 29" bike to a person for a ride. Someway he ruined the freewheel (it had a shimano freewheel) - the 2 smallest cogs were badly slipping. Now I put another freewheel (also shimano. it's second hand, but I tried it, and it was in perfect condition). I gave the bike to another person with the "new" freewheel, and by half a hour he ruined this freewheel - the smallest cog is badly slipping, and the second (i.e. #6) a little slipping too.
So here is the question: does the wheel size affect so much, to crush the freewheel in half a hour?
Edit: The bike was new. Just a few rides. The chain length is good - the derailleur is at right position. And yes, it is a freewheel, not cassette.
What caused me to think that freewheel ruined, is that I saw him using cog #7 at rear, and #1 at front.
As mentioned, on first freewheel 2 smallest cogs were slipping. Others are just perfect. Can it be because the chain is ruined as @hillson wrote?