Image Quality Issue

I have image quality set to 100 in hopes to have the best quality images I can possibly provide for my users. It appears that their is a big reduction in quality with the resized photos. Compare the original with the large. In the large you can see graininess above the mountain, loss of sharpness, big grainy blocks, and loss of detail. But the stock photo looks fine.

We both know that resizing a photo typically reduces the quality, however I have a powerful counter argument in this case to that being the real reason for the quality reduction. Here is ES's 1280px photo and here is one I manually resized to 1280px. When looking at the file sizes the lower quality one is 68.3 KB while the other one is 251 KB. So I know for a fact that the resize photos are losing a lot of details, more so than what is ideal. It's not just sharpness we are losing, I can see the graininess in the processed photo. In my manually resized one it has very little grainy details.

Is this my image processor's fault? Does this happen on your dev site? Is there some sort of compression going on?