[W]hen considering a jpeg image’s file size, the level of compression makes more of a difference than its physical dimensions. In other words, given two identical images that are displayed at the same size on a website, one can be dramatically smaller than the other in file size if it is both highly compressed and dramatically larger in dimensions than it is displayed.
If you want to display an image at 400 pixels width and 300 pixels height, saving it with those dimensions and displaying at 100% size, with a typical 90% quality jpeg compression from Photoshop, weighs 95kb.
The same image, however, saved with the dimensions 1024x768px and scaled down by the browser, with a 0 (zero!) quality jpeg compression from Photoshop, weighs 44kb!
And will then scale very well, without compression issues…