When two, line-break, <br /><br />, tags are used in wikitext, then for smaller font sizes, the line break produces too-large a vertical spacing between the lines. The effect doesn't depend on which Wikipedia skin is in effect. Here are some examples to try on the wiki:
This is a paragraph with default font size.<br /><br />Note the acceptable paragraph spacing between this line, started with two line-breaks, and the previous line; both inside a normal font. And I'm hoping if I add enough text this line will wrap to show that two paragraphs really can be in one line.
<font size="-3">This is a paragraph with a <font size="-3">.<br /><br />Note the unacceptably large spacing between this line, started with two line-breaks, and the previous line. It's the same spacing as in the first example. It should have becoming proportionally smaller vertical spacing because the font is smaller.</font>