What is the logic behind the unequal divisions of Tehillim into days of the month; the sections are of very varying lengths i.e. the "longest days" are twice the length of the "shortest days?"
The division of the Tehillim is not meant to be just a way to say the entire Tehillim over the course of a month, rather it was split like this because each day of the month is different and has its own path to follow in the service of Hashem. Each days chapters' are special for that day.
Similarly, the Psalm of the day (which the Leveim sang in the Beis Hamikdash) for Wednesday is at least twice as long as the one for Friday. Each day is special and has a special way to serve Hashem.