If you set your output profile to Tablet then the images are not resized at all, so if they are too small, then they would have been too small int he source. Remember that simply setting the option in Preferences->Page Layout is not enough since previously used settings take preference.
http://manual.calibre-ebook.com/conv...for-conversion
So fix the settings and reconvert, that will be the easiest way to resize images in bulk.