IMO two different things can make a game heavy, and having either is enough even if you don't have both.
First, is complication of decisions. This is the classic "heavy" angle, how much thinking you have to do each turn to execute your strategy.
Second, is the actual meta aspects, such as rules, or pieces, or setup. A game that is literally heavy by weight is highly likely to be heavy, especially if combined with lots of rules or pieces or exceptions. Even if the turn-to-turn is relatively simple, the act of setting everything up and having to learn & remember a whole slew of rules can make a game heavy