Chain length doesn't affect the chain's traction on the sprockets either way. If you think about it for a second you'll see that. The chain on top is tight reflecting the bike driving force, but the bottom loop is basically slack, having only the tension from the RD idler cage spring. If the sprocket didn't keep the chain engaged, it would pull the bottom loop around all the time regardless of chain length.

So you need to look elsewhere for your answer. I suspect that the RD tab may not be aligned perfectly so the trim changes slightly as the RD pulloy swings it's arc. of if this is a Shimano or Campy derailleur (doesn't apply to Sram) the upper pulley rises and falls with RD cage rotation. That means the B-screw adjustment needs to be checked in every gear combination. If you didn't do this there's a decent chance that when on the smaller front sprocket, the idler cage rotates, bringing the pulley high and into contact or near contact with the sprocket. On a hunch, without seeing your bike, bring the B-screw in to lower the RD a bit and see if that magically clears the problem.