Your shop is being browsed on all manner of devices from small-screened mobile phones, through tablets and up to large desktop displays. It should look great on all of these and load fast.
The way to achieve this is with responsive images. Your shop's theme can be coded in such a way that visitors will load small sized images on small device screens and larger sized images on larger screens. This ensures there is no time wasted downloading big images for small screens: a faster browsing experience for your customers.
Most themes are now responsive, meaning the content reorganizes itself depending on screen size. This does not mean that your theme has the responsive image loading feature. It may be displaying a smaller image on a small screen but still downloading the larger image.
Speak to your theme developer about responsive image loading and if it is possible on your theme. There are lots of good resources about implementing responsive image loading on Shopify. Do CC us at firstname.lastname@example.org if you email your theme developer- we're always interested in helping out where we can to speed up your shop.