Why this isn't actually a smaller font
Social media bios, chat apps, and most comment boxes don't let you change font size at all - so tools like this work around that limitation by swapping each letter for a different Unicode character that happens to look like a smaller or raised version of it. A small-caps "a" generated here is really the separate Unicode letter "ᴀ" (borrowed from the International Phonetic Alphabet), not a resized ordinary "A" - which is exactly why it can be pasted anywhere plain text works, with no special formatting support required from the app you paste it into.
Small caps vs. superscript vs. subscript
Small caps renders your text as uniformly small capital-style letters, popular for stylizing a username, bio line, or section heading. Superscript raises each character up and shrinks it, the same visual style used for footnote markers or exponents in math - useful for a distinct, attention-grabbing effect in a caption or comment. Subscript lowers and shrinks each character instead, matching how chemical formulas (H₂O) or mathematical subscripts (xₙ) are conventionally written.
Why a few letters stay normal size
Because this relies on finding an existing look-alike character already defined somewhere in the Unicode standard, a handful of letters simply don't have a good matching equivalent in every mode (for example "q" in small caps, or most consonants in subscript - Unicode's subscript set is much smaller than its superscript set, covering mainly digits and a limited set of letters used in scientific notation). Rather than substitute something that wouldn't actually look right, those letters are left in their normal size.
Where this works - and where it might not
Because the output is just Unicode text, it pastes correctly almost anywhere plain text is accepted - Instagram and TikTok bios, Discord, Twitter/X. A small number of older or very restrictive apps may not render every uncommon Unicode character correctly, in which case a letter might show as a blank box.