In everyday life
A building inspector can check a small extension room by room in an hour. Given the same hour for a whole new tower, they walk the halls and sign. A small request gets a real inspection.
One case
A developer on an airline booking site opens a pull request that changes 1,900 lines across 46 files. A reviewer reads it for 25 minutes and approves it. Line 1,312 removes the check that held refunds over $500 for a manager. The reviewer never saw that line, and the first large refund goes out unchecked a week later.
A request to merge a branch into the main line. It shows the changes, usually runs the automatic checks, and gives teammates a place to review and comment before anything joins.
Every case
A pull request is where a change gets a second pair of eyes and the automatic checks before it reaches everyone. That works only if the reviewer can hold the whole change in their head. Small pull requests get careful reviews. Large ones often get skimmed and approved, because reading 2,000 lines with care takes hours that nobody has. The size of the diffThe lines a change adds and removes, shown side by side. Reviewers read the diff to see what changed. sets a limit on how good the review can be.
The word
Engineers call this
Keep each pull request to one purpose, small enough to read in one sitting. Say what it changes, why, and how you checked it.