In other words, the 16" has a battery that's 70% larger.
Display power consumption scales with area, and a 16" is around 50% larger (the diagonal to area relationship is quadratic, not linear).
For one, RAM. M1 machines use two RAM modules (with separate channels). Whatever SoC goes into the 16", it will ned to use at least four (or maybe even six) independent RAM modules/channels if Apple want's to offer a competetive graphics solution. Then, the SoC itself will use more power (larger caches, linger/wider interconnect etc.). 3x3 Wifi instead of 2x2. More Thunderbolt repeaters. Maybe some other things I am missing (especially on the power delivery side).
I think the increased baseline power consumption from factors I listed about is about right to eat up that 70% battery capacity advantage
As I wrote before, I'd expect around 22 hours battery life, but not more.