Hi L52509,
This is actually a somewhat common issue that usually stems from the original file size of the images rather than how they look in the editor.
Here are a few things to check:
Some email clients (especially Outlook for desktop) often ignore the size you set inside the email editor and instead display the image at its full, original pixel width. Try resizing your image in a photo editor to the exact width you need ( 600px or 300px wide) before uploading it to Constant Contact. This forces the email client to display it at that size.
Which email program are you seeing the size difference in? Knowing if it's Outlook, Gmail, or Apple Mail can help us narrow it down further!