@ssun30 if Toyota wanted, Prime could be a lot faster... it uses 71hp from batteries in EV mode, right? While regular Prius uses something like 22hp from batteries? But they focus on MPG, not power so they dont combine the both for maximum acceleration.
The Prime's battery pack is capable of discharging at 80kW while the total motor output in EV mode is 71kW by combining the power from both MG1 (22kW) and MG2 (51kW) via a sprag clutch in the P710 transaxle. This operating mode is not available in hybrid mode which means MG1 cannot add power by draining the battery, serving only as the generator and engine speed regulator. The hybrid transaxle on the Prius is very optimized that components are almost sized to exactly match each other with pretty thin margins (outside of the inherent reliability margins, of course). This allows a very compact, lightweight, and efficient hybrid system at the cost of flexibility. There's very little power to gain (a few kW at most) from the extra battery power so they just rate the Prime the same as the regular Prius. The hybrid systems in the UX and the Camry, on the other hand, do have some healthy margins for more power capacity so they could get an uprating with a more powerful battery pack.
And no it has nothing to do with MPG. If anything having more power to the motors benefits MPG because it reduces the burden on the ICE.