In the default theme, and others as well, the font size of the equations are too small, smaller than the font size of the theme itself, while it should be equal.
To fix this you could use the following snippet. Even when selecting different font sizes through the editor, the font sizes keep matching. (It could be that the 1.25 should be changed for some other themes.)
mjx-container { % works for live preview and reading mode
line-height: 0px;
font-size: calc(1.25 * var(--editor-font-size));
}