Short answer
HubSpot's KB: images on HubSpot-hosted content are resized by default to fit the screen, which can cut load time and help PageSpeed. Images with height and width attributes in <img> tags are resized. Add ?noresize to the image URL to turn it off for that image.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Only turn it off for images that look wrong; resizing helps speed.