its because your wp theme has some extremely greedy css rules…

*:before, *:after {
    box-sizing: border-box;
}
*:before, *:after {
    box-sizing: border-box;
}

basically that makes all flexbox be border-box…  so it affects many, many elements and limits the size…

to correct, see:  https://simple-press.com/documentation/codex/installation/changes/theme-and-templates-5-5-4/

the area on box-sizing: content-box

you will want to add that to the main css declaration in your theme – looks like this:

#spMainContainer div,
#spMainContainer span,
#spMainContainer h1,
#spMainContainer h2,
#spMainContainer h3,
#spMainContainer h4,
#spMainContainer h5,
#spMainContainer h6,
#spMainContainer p,
#spMainContainer blockquote,
#spMainContainer pre,
#spMainContainer code,
#spMainContainer em,
#spMainContainer img,
#spMainContainer big,
#spMainContainer small,
#spMainContainer strong,
#spMainContainer b,
#spMainContainer i,
#spMainContainer ol,
#spMainContainer ul,
#spMainContainer li,
#spMainContainer fieldset,
#spMainContainer form,
#spMainContainer label,
#spMainContainer input,
#spMainContainer select,
#spMainContainer legend {