thumbnail sizes

I'm having some confusion about the size of image being loaded for my thumbnails. I have various sizes available, but it looks like the largest possible size is being used. So on Chrome I'll see something like:
"380 x 285 pixels (intrinsic: 2600 x 1950 pixels)"

Does this mean that all users are getting the huge image? Or am I misunderstanding "intrinsic"?

Please look at the grids under the "recent posts" heading on the front page.