How the numbers were measured
Every script was downloaded from the exact address its vendor tells you to paste into your page, with compression turned off, and then compressed here: gzip at level 9 and Brotli at quality 11. A CDN can send the same file a little smaller or larger depending on its settings, so measuring them all the same way is the only fair comparison. A kilobyte here is 1,000 bytes.
The script that measures them is in the invisit repository, and the page shows the day it last ran. If a vendor changes its script, the numbers move on the next run.
What the size does and doesn’t tell you
A smaller file downloads faster, but download is the cheap part. On a phone the cost is mostly parsing and running the code, which happens on the same main thread as your own scripts. A 150 KB analytics script can show up in Interaction to Next Paint on slower devices even when it loads with async.
Size also isn’t the whole bill. Some tools load more once they run: session replay recorders, surveys, heatmap modules, or other tags pulled in by a tag manager. Where a vendor documents those extras, the Markdown copy of this page lists them.
Check your own site
Open your browser’s developer tools, go to the Network panel, filter by JS and reload. The Transferred column is what came over the wire, Size is what the browser had to parse. Do it once with a throttled CPU and a mobile profile and you’ll see which of your scripts actually cost something.
invisit itself is on the list, measured the same way as everyone else. If size is your main concern, some tools here are lighter than ours, and that’s fine to choose.