I don't play HU but tried to play one game at 10NL. I can't remember exact numbers but we both started with 10$ stack and played many hands until I took his all stack. I left table with ~14-15$. Is it just seems to me that is harder to beat rake at HU then full or 6max ring because money is constantly changing owner and every pot is raked.
The more players, the more money on the table. The rake is less noticed.
Be aware that:
The rake is increased together with the number of the hands. If you play HU and on the first hand you both go all-in, the rake will be minimum. If you play many hands, the total rake is increased (and of course, the amount of money is decreased).
If you play in a full table, the behavior will be the same, but you less notice it. Another issue is that in full tables, players leave and new players join (the table always keep on a certain level of "total money").
Well, think about it: if you're heads'up, then the rake is basically from both player's stack since in most heads'up matches, almost all hands are played (super wide range).
If you are in a full ring game (9 players), then maybe you play AT MOST 25 % of the hands. In this case, only in that small percetange of hands rake is taken from your stack / pot. So you will feel the rake a lot later. Also, in full ring games, there are situations (not that rare) where more than 2 players go to the showdown with their hands, which means rake is taken from all of them.
You're probably better off playing in a head's up Sit 'n' Go game. Then, it won't matter how many hands you play because you simply pay the one-time tournament fee.
For example, on one of the online poker sites, the $10 head's up SnG game is $10+$0.50. You and your opponent pay $0.50 each as "rake". The winner gets the loser's $10 no matter how many hands you play. In your case, you would've won an additional ~$5-$6 if you had gone this route.