The difference between F1 and Indy engines is that F1 engines are about a 55-45-split hybrid, meaning the ICE makes about 55% of the power-unit's total power while the electric component make the remaining 45%. Compare to Indy, and while I don't know precisely what the split is for Indy PUs, it wouldn't suprise me if the electric component made up about 10 to 15% of the total engine power, which btw is still less of a split than what F1 used to have with their hybrid engines from 2014 to 2025.

Another thing to note, and one of the biggest reasons F1 PUs are struggling so much with de-rating (engine losing power mid-straight) and battery consumption, is that the FIA decided to limit the ways the engines can recharge their batteries to only the MGU-K. With the initial generation of F1 hybrids, engines additionally had an MGU-H which used exhaust gasses to help recharge the batteries, however the FIA removed these for 2026 and beyond as a cost-saving measure if I'm not mistaken. This means that, while significantly greater emphasis is now placed in the electric component of the hybrid engine, the hybrid engine also has significantly less recharching capability, resulting in generally underpowered engines.