The large file sizes are probably due to a direct conversion from your presumably compressed tiffs. Try reducing the resolution and size, if that's appropriate for your application. Images for the web probably don't need more than 100 dpi.

If you wish to consider offline batch processing of all images, I've used IrfanView in Windows. This can do batch conversions from a command line (and no GUI appears). Never needed to do this from Unix/Linux (or modern Mac) but Gimp or the stand-alone ImageMagick can probably help.