The size of a diff tells you how long it takes to read. It doesn't tell you how much to worry.
Blast radius
How far a change can reach: everything that imports it, calls it or relies on how it behaves, directly or through something else. A one-line change to shared code can have a bigger blast radius than a thousand new lines nobody uses yet.
An agent sees the file it is editing far better than the callers it hasn't opened, and a reviewer reading the diff sees only the lines that changed. Nobody in that exchange is looking at the code that will really break, unless someone goes and looks.
“Before you change formatMoney in src/lib/money.ts, run rg "formatMoney" and list every caller. Pick the riskiest one, write a test that pins how it behaves today, and run that test before and after your change.”