The tag view uses the "generic" size referenced by default in K2's parameters, under the Images tab in "Item generic image width (in px). Used in user pages as well as tag, search & date listings." Set the right pixels there and K2 will make sure to resize images properly for this view. If you had this setting tweaked to a small value, then uploaded your K2 items and then you changed the value back to a larger dimension, then K2 will only "HTML resize" the images. To fix this, you need to re-upload your images.