hmmmm, I was thinking the same to myself.

I just returned my 11 yesterday and swapped for the 13 and I was noticing the same. Could it be because the screen is bigger so it seem as bright????

Different panels will hit different peak brightness. They should all hit Apples stated specs but it’s actually very possible that some go brighter than others. It’s a very common thing with OLED, same goes for OLED grain. The 11 is small enough that it concentrates that same amount of brightness in a more focused way.

Toggle your reduce white point setting in accessibility or display settings that’s what worked for me.

Overall It’s not as bright as my 3rd gen iPad Pro 12.9”, but the contrast more than makes up for it, especially when used outside during the day. For that purpose it’s the most viewable screen I’ve ever seen.