Three kinds of small text
Superscript (ᵗⁱⁿʸ) sits high on the line, like an exponent. Small caps (ꜱᴍᴀʟʟ ᴄᴀᴘꜱ) are capital letter shapes at lowercase height. Subscript (ₛₘₐₗₗ) hangs low, like the 2 in H₂O.
None of them is a smaller font size. Each tiny letter is its own Unicode character, mostly borrowed from phonetic notation, where a raised ʰ marks a puff of air after a sound. That is why little text keeps its size when you paste it into a bio, caption, comment or game name where you cannot change the font.
Why some letters stay big
These characters were added to Unicode for phonetics and math, not as a tiny alphabet, so the generator fills the gaps with look-alikes. Superscript q is the Canadian syllabics character ᑫ, and a few capitals use small forms (ᶜ, ᶠ, ˢ) or stay as they are. Small caps take most letters from phonetic alphabets (ʀ, ɢ and ʜ are IPA letters), with a plain x and the hooked ǫ standing in for X and Q.
Subscript has the biggest gaps. Unicode has subscript forms of only 17 letters (a, e, h, i, j, k, l, m, n, o, p, r, s, t, u, v and x), so letters like b, c and d stay full size. Superscript digits can look uneven too, because ¹ ² ³ come from an older block than ⁴ to ⁹ and some fonts draw them differently.
Where tiny text fits
Small text works best as an accent: a quiet tagline under your name, a ᵗʰᵃⁿᵏ ʸᵒᵘ at the end of a caption, or a footnote-style aside. For math and chemistry, exponents and formulas look cleaner with real superscript symbols and subscript symbols, such as the squared symbol ².
Tiny letters are hard to read on a phone, and screen readers may announce them by name, such as "modifier letter small t", or skip them. Keep anything important at full size. For bigger effects, try the fancy text generator.