Why a large file stops part way through the preview

You press Space on a large log or a generated file, read a few hundred lines, and the preview stops with a note saying it was truncated.

That is deliberate, and it is worth understanding why before you go looking for a setting to turn it off.

Why a limit exists at all

Quick Look is a glance. macOS gives a preview extension a short window to produce something, and the whole point is that you press Space, look, and press it again.

Syntax highlighting is the expensive part. Every line has to be tokenized against a grammar, and on a multi-megabyte file that work grows with the file while the useful information does not—nobody reads line 40,000 of a log in a preview pane.

So Syntax Look renders the first 512 KB and tells you it stopped. The alternative is a preview that spins, or one macOS gives up on and replaces with a generic icon, which tells you less than a truncated preview does.

The banner is the feature

A truncation banner is the honest outcome. A preview that quietly showed you the first part of a file with no indication would be worse: you would scroll to what looks like the end and conclude something is missing from your data rather than from your preview.

When you need the whole file

For a genuinely large file, reach for a tool built to stream it.

less +G huge.log          # jump to the end
tail -n 200 huge.log      # the last 200 lines
grep -n "ERROR" huge.log  # find what you are actually after

For a large source file, an editor. Quick Look was never the right tool for that job, and a preview that tried to be would be slower at the job it is good at.