You're kind of right and kind of wrong. League resets after every tackle that means you don't need muscle (big forwards) to secure the ball. Because you are limited to the number of tackles before you have to turn over the game has more running and passing in an attempt to break to defence. The resetting means it doesn't "flow" as much as union, however it is more run oriented so from that perspective it flows better.

Union on the other hand relies heavily on the forwards (obviously depending on team style). The job of forwards is not to gain yards but to suck in the defensive line. When a big guy goes you have to commit to tackling him, the more you commit the thinner your defence is on the edges. That's why you see phases of forward play to give yourself an advantage elsewhere on the field, when you feel you have enough of an advantage/see a hole , you pass out to the backs and they make a run.

Also union (especially the forward roles) is a lot more focused on forcing mistakes. There is a lot of technical stuff going on when someone goes to ground or when a maul is formed. Forcing a mistake is a huge part of gaining an advantage.

Lastly no serious team has overweight forwards , even the barrel chested short dudes are anything but overweight and in fact probably do the most work in an average game. Watch a game of union, every breakdown there is at least two forwards there rucking over the ball , pushing people off , tackling.