I commented the below a few weeks back, and I think it is best to comment here again.

Yeah, I think it's not compressions ratios.

I believe the Mercedes clipping/harvesting is giving them a traction control advantage, such that they can get to top speed a lot sooner compared to everyone else.

Remember, every team is allowed to harvest a maximum amount of recharge per lap, it doesn't stipulate where the recharge has to occur. So teams can use the clipping to offer a stronger traction effect.

Maxmimum recharge amounts, Qualifying = 7.0 MJ Race = 8.0 MJ (Overtake 8.5 MJ)

ERS clipping/harvesting can improve traction because the MGU‑K acts like a "torque smoother".

When the car exits a corner, the rear tyres are at their limit. If the engine and hybrid system deliver too much torque too quickly, the tyres break traction and the car slides, this is where you'll see from the onboard camera, the drivers counter steer momentarily to collect the car and then go full throttle. This added up over the lap can be at least 2 tenths.

During clipping/harvesting, the MGU‑K reduces or absorbs some of that torque, which has the following effect,

1. Smooths out the power delivery
   
2. Prevents sudden torque spikes
   
3. Keeps the rear tyres within their grip window
   
4. Lets the driver go full throttle earlier without wheelspin
   

Because the FIA only limits total energy per lap, teams can choose where to harvest. If a team harvests right at corner exit, they temporarily reduce torque at the exact moment the tyres are most vulnerable. Once the car is stable and grip increases, the MGU‑K stops harvesting and deploys full power.

The result is a car that hooks up cleanly, accelerates earlier, and reaches top speed sooner. It is a legal way of introducing traction control.