That seems to be generated from Openstreetmap data via a library called
Leaflet. If I search for that there are a variety of other postcode maps
available:
https://www.freemaptools.com/uk-postcode-map.htm
https://www.aj2duncan.com/blog/postcode-polygons-on-a-leaflet-map/
It might be possible to make similar to streetlist with a bit of tweaking.
Another option is to load up the streetlist page, right click over the map
(I'm in Firefox), go 'Inspect', and look for
<div id="map" style="width: 100%; height: 500px; border: 1px solid rgb(170,
170, 170); position: relative; outline: currentcolor none medium;"
class="leaflet-container leaflet-touch leaflet-retina leaflet-fade-anim
leaflet-grab leaflet-touch-drag leaflet-touch-zoom" tabindex="0">
If you edit that (double click that HTML line) to something like
width: 2000px; height: 1000px
you get a bigger map. Of course, it's just for you and won't persist over a
reload.
Theo