Start with a "rules lite" game system. The system covers 80% of a player's game experience.
Group 1:
When the players want to try stuff not covered by the system, the DM has to come up with rules for it. Once a rule is formed, the DM adds it to the list of house rules for future reference, should the situation come up again.
After 100 game sessions, that 20% has come up quite often. The DM's house rules list has grown quite a bit. The DM's game is now rules lite (80%) with house rules (20%). He looks at the "rules heavy" game system and sees that all of his house rules are included in the core rules (100%). So the group "upgrades" to the new game system.
The group is happy with the new rules because nothing has gotten harder, and the DM's job has gotten easier.
Group 2:
Start with a "rules lite" game system. The system covers 80% of a player's game experience.
Either the players never think of or attempt something not covered by the system (the 20%), or they consciously or subconsciously limit themselves to stuff covered in the system (the 80%). [If grappling is not mentioned in the rules, do the players not think to try it, or do they ignore/avoid that option?] So the "missing" 20% is never actually missed.
Someone sees the "rules heavy" game system, sees all these new options (the 20%) available, and the group upgrades to the new game system. Now everyone wants to try all these new options. The group no longer just attacks, attacks, attacks. Now everyone is bullrushing, grappling, and disarming. The 20% suddenly becomes so much more important. The 20% seems more like 50% now.
Everyone starts pining for the old days when things were simpler. "Rules lite worked fine for us."
So, for Group 1, "rules heavy" was a good change. For Group 2, "rules heavy" was a bad change. One group saw the old cup as half empty, and now sees the cup as full at last. The other group saw the cup as having plenty, and now sees the cup twice as full as it needs to be - and they feel that they *must* drink the whole thing.
Quasqueton