Using image transforms sometimes results in bloated file sizes. I'm in the process of setting up Asset image transforms for my site and was playing with the quality setting to find the right balance between file size vs. output image quality. 

As part of the testing process I was outputting matching images at the same final transform size in Photoshop at various levels of "Save for Web..." quality. I was surprised to see that the images Craft was generating were consistently larger in file size  than the images Photoshop was generating with approximately the same final visual quality. Typically the difference was anywhere from 20-40%. 

It feels important to note that I realize that Craft's image transform quality settings don't equate to Photoshop's "Save for Web..." quality settings (i.e. using a quality setting of 65 in Photoshop is not the same as using an image transform quality setting of 65 in Craft).

I tested a variety of image types at a variety of transform and quality settings and found the results to be roughly the same in every scenario. 

I'm curious if anyone else has seen / noticed this issue and if there might be a fix.
Is Imagick installed on your server?

If not, Craft uses GD as a "fallback". Chances are that transforms with GD lead to poorer image quality and longer calculation times. I'm not sure about ICC profiles, but I think they only get included using Imagick (photos can look really bad if profile is missing unexpectedly!).