When you're resizing images in Outlook, that's actually a feature of Outlook and not Snagit. Unfortunately, Outlook:Mac isn't as fully-featured as it's Windows counterpart, and I've run into the same trouble myself.
What I typically do, is scale down the size of the image in Snagit on Mac before I paste it into Outlook. You can do this in the Snagit Editor if you look under the Edit menu there's an option for Resize Image and you can just scale the image down by a percentage, I usually do 30 percent.
I hope this helps!