Less pixels means less data to store.
Smaller image pixel dimension will result in a smaller file (kb).
(Note: There is no direct correlation between the amount of pixels within an image and file size of the image - actual pixel data for a single pixel will vary in size. Data size is not a constant factor. Some pixels will need to store more data than other pixels.)
Each pixel is a data point. The more pixels there are the more data points there are. It doesn't matter if there are 10k pixels displayed at 100x100px or displayed at 500x500px, it's still the same 10k data points.
CSS doesn't do anything in terms of image compression (kb). CSS just sets display options.
The difference is that using CSS to reduce the dimensions can allow for images to display better on Retina or 4K monitors. Higher resolution monitors display higher pixel count images better. But again, there's no compression with CSS.
If one has 2 images, for example: One image is a 10,000 pixel image (100x100px) and the other is a 250,000 pixel image (500x500px) - Both images are displayed at 100x100px on a web page. Even though both images are set to display at the same pixel dimensions, the 250K pixel image is going to look much better on a Retina/4K monitor compared to the 10k pixel image.