A JPG may occupy only a few megabytes on disk while expanding into millions of pixels in memory. The browser needs decoded data to draw a canvas, create a thumbnail and encode a new result.
The Image Prep tool limits files to 24 megapixels, 20 MB per input, 20 files and 100 MB total input. The limits reduce risk; they cannot guarantee a successful run on every phone or low-memory device.
A safer batch routine
Test one landscape and one portrait image first.
Process smaller batches when the device becomes slow.
Reduce output dimensions before assuming a format change will solve memory pressure.
Keep originals outside the browser and inspect every downloaded result.