This tool maps ordinary letters and digits onto the Unicode "Mathematical Sans-Serif Bold" block, a range of look-alike characters originally intended for math notation. Because each character is a genuinely different code point with heavier strokes than plain text, it visually reads as "bigger" and punchier in a bio or title even though the font size hasn't changed. Since it's real text rather than an image, it stays selectable, searchable, and copy-pasteable, though a handful of very old apps or fonts may render a fallback box for unsupported glyphs.
It's a different character shape, not a font-size change — the Unicode sans-serif bold characters simply have heavier, chunkier strokes than normal letters, which makes them read as "big" at any size.
Usually, since these are still Unicode letters — but a small number of strict username fields only accept plain ASCII and will reject or strip anything outside it.
Digits do map to bold Unicode equivalents, but many punctuation marks have no dedicated bold Unicode counterpart, so they're left as the original character.