I have my AudioMoth configured to sleep 5 seconds and record for 55 seconds from 4am to 8am every morning. I end up with 240 files to evaluate each taken every minute. However, today, for the first time, I had files of 5.3 MB size from 4am to 5:30am and then every file after 5:30am of completely varing sizes and some zero bytes. When I load the files after 5:30am, they are all different lengths of time.
This sounds like the battery voltage was getting low and the AudioMoth started to stop recordings early due to the voltage dropping below the minimum threshold. If you check the comments in the files (you can normally see this within a WAV viewer such as Audacity or on a Mac you can see it in the Finder) it will give the reason for the short file.
The minimum supply voltage threshold is 2.8V and this typically results in a minimum battery voltage of about 3.2V.
Did you accidentally turn on "amplitude threshold recording"? That would explain getting recordings of different lengths. See here: https://www.openacousticdevices.info/config-app-guide
Wim