This happens because you’ve added a rule to make your image grid full-bleed:
@media screen and (min-width: 990px) {
...
.collection--full-width slider-component:not(.slider-component-desktop) {
padding: 0 1.5rem;
max-width: none;
}
}
max-width was initially 120rem, you’ve set it to none.
However, your collection title element still has this width-limiting rule. This is why when you make your browser wider, collection grid becomes wider, but collection title stops growing after 1200px (which is the equivalent of 120rem in this theme).
And here lies the reason why your images become blurry.
The theme uses srcset image parameter together with sizes. These two allow browser to select image which will be the best match for current browser window width and sizes is the parameter which defines how wide the image will be give the window width.
Here is the sizes for your featured collection product images:
sizes="(min-width: 1200px) 267px, (min-width: 990px) calc((100vw - 130px) / 4), (min-width: 750px) calc((100vw - 120px) / 3), calc((100vw - 35px) / 2)"
It says that if window is wider than 1200px, the image size is expected to be 267px, which is no longer true – your images will grow wider since you’ve unset the max-width rule.
For your setup the sizes parameter should be similar to:
sizes="(min-width: 990px) calc((100vw - 2*1.5rem - 2* 8px) / 3), (min-width: 750px) calc((100vw - 128px) / 2), calc((100vw - 35px) / 2)"
Unfortunately, since themes switched to native responsive images from lazysizes library, calculating proper sizes parameter became a quite complex task.
Because this code is good for 3 images per row on desktop, but if you decide to switch to 2 or 4, it will become wrong.
Therefore it’s a bit difficult to properly fix your problem by editing Liquid code.
I would try this – add a “Custom Liquid” section aright after the “Featured collection” one and paste this code: