How can I decrease the size of a UI Tab Bar Item Image so that it fits inside the Tab bar?
This is not possible.
Check Icon and Image Sizes guidelines from apple. It categorically mentions:
Unlike other custom artwork in your app, the icons and images listed in Table 41-1 must meet specific criteria so that iOS can display them properly.
This applies to the tab bar icons too. So, if your images are larger than the max size (mentioned in above link), you will face such issues.Better resize the images.
If you are worried about poor resolution on retina displays (6, 6+ etc), make sure you provide @2x and @3x images for each tab bar image to Support the Retina display.