My slider's images have larger file-size than their originals I have a slider in my homepage and this slider has 3 photos named a.jpg, b.jpg, and c.jpg.

The original file's sizes are (respectively), 42kb's, 20kb's, 52kb's. And yet when I download them from my slider (Flexslider) they whey much larger (c for example is 191 kb's).
Replace image style from the Slider's View (At Views module). I kept it without any style so the image won't be scaled to a larger style (You could scale it yourself with CSS in the relevant Div at the slider and it won't affect size).