I don't have Photoshop (no need for it), but I do use
Irfanview.
To make thumbnails with Irfanview, first open it up and select "Batch Conversion/Rename" from the File menu.
Then navigate to the folder that contains the pictures you want to make thumbnails from, select all the pictures and click on "Add" in the Batch Conversion dialog that comes up.
Then, check "Use advanced options" and click on "Set advanced options". Check the "Resize" option and click the "Set new size" radio button. Click "Set both sides to" and enter the width and height you want the thumbnails to be, ensuring the pixels radio button is selected.
Then, just click on Ok and then once your back to the Batch Conversion dialog, click on "Start". A new popup should come up with the progress of each of the conversions and if they all go smoothly you'll be able to see them in the conversion folder. I believe the default conversion folder is "C:\Temp". If the thumbnails are too large, try repeating the procedure on the original files but go into the "Options" on the Batch Conversion dialog and adjust the "Save Quality" to a lower value.
The reason PNG files are (generally) larger than GIFs is that while both are lossless, GIF uses 8 bits (256 colours), whereas (IIRC) PNG uses 32 bits (4294967296 colours). PNGs are almost always larger than JPGs too, because PNGs are lossless but JPGs are lossy.