Kerning

Fonts ship with default spacing tuned for body copy at reading size. Blow the same word up to fill a thumbnail and specific pairs come apart: a capital A next to a V, a T followed by a lowercase letter, anything either side of a comma.

At the size a thumbnail is actually seen, this decides whether the word reads at all. Cramped letters merge into one shape on a phone feed. Spacing that is too loose breaks a word into pieces. Shrink your canvas to match a mobile feed and fix whatever collides at that size.