You could try to Cut the grouped images and then use the Paste Special option to paste back as a Picture.
You can then use the Format (Picture Tools) tab, when the group is ‘active’ and use the Crop Tool to remove the excess/not required 'oversize'.
Click the image below to see a short video of how I have achieved the above.
I haven’t spent much time on precision cropping to cut down the video length, but zooming in lets you crop a little more accurately.
I have only used the centre bottom ‘handle’, but the side, corner and top handles can also be used.
You can then Save As the cropped grouped image as a PNG or other format. I have used GIF in my example.