Alignfull

I got this code from the web

.entry-content .alignfull {
margin-left: calc(50% – 50vw);
margin-right: calc(50% – 50vw);
width: 100vw;
padding: 25px;
}`

It is working OK.
I don’t understand why it needs the padding: 25px to offset the margin.

But in any case, when the vw is narrow, the fixed 25px feels bigger as the screen gets smaller.
Any better code than this?