Today I increased review frequency from "normal" to "frequent." My kanji per lesson went from 18-22 max to 27.
I haven't touched the setting for "kanji per lesson."
I think this is expected. The number of kanji per lesson is based on how easy those kanji are for you. When lots of kanji in the lesson are hard, you'll have fewer kanji per lesson. When lots are easy, you'll have more kanji.
When you make reviews more frequent, you'll necessarily be making easier kanji more common, so I'd expect lesson batches to be larger.
I have plans in the future to make this process more transparent.
