When the load changes from light to heavy, while the input current is less than the Ipeak, the part will work as boost.
When the input current increase further, and the duty cycle up to about 75%, MAX1722x…
The best answer / explanation for this can be found in the datasheet:
PFM Control Scheme
The MAX17220/2/3/4/5 utilizes a fixed on-time, currentlimited, pulse-frequency-modulation (PFM) control scheme…
