I don't believe they are cheating by adding small cards.
You can easily test your theory by counting down the cards. Maybe count down the number of aces as well.
Even if you find small card is a little more, due to experimental chance, it doesn't really matter. If you are AP, you advantage should be at least 0.5% to 1%. So for 6 deck, they need to slip in 6 small cards to break even you.
Why cheat in BJ? risk your reputation on a game no one plays anyway, and risk scare off bacarret players? To counter AP, can just ban us, or no dead chips.
btw, if you have a good game and don't want other people coming around, is to scare them off smartly...