We're probably talking about different issues then, as it sounds odd I think unless your large high-res images are just too large that they take too much RAM when loaded. Note that all compressed images are shown on screen uncompressed by browsers, etc, so even though your JPG image may be 1mb each (which would be too large for a 360 product image file already by our standards) the uncompressed size is usually much larger.. So if you have a bunch of really large high-res images, when they are loaded and uncompressed they may easily consume a lot of RAM (it will depend on how particular browser will deal with images that are loaded but not visible at the moment as you move from one image to another).
When you select "create an extra set of small fitted images" in SpotEditor, the software creates another set of small images that it auto-sizes to fit into your chosen viewer dimensions exactly and they are the ones loaded on page load (they are usually quite small). The high-res ones are only loaded on demand in this case when user hits zoom or full-screen. So if you don't check the checkbox, the high-res ones are loaded on page load instead, and it's usually not recommended as it's a much slower option.
To see exactly what's going on with your setup though, please send us a live link so we could review and comment.