The Google Analytics script itself, analytics.js, is cachable and cached.
By design, Google Tag Manager and gtag should not be cached. The behavior of those scripts can be configured from wizard interfaces, and there is an expectation that published changes take effect immediately.
Also, all of these scripts are deeply and thoroughly asynchronous. They are designed not to impact site speed. Not to say that they can't or won't (I've seen GTM do horrendous things when used improperly), but I would trust an actual timing metric over a file size metric.
Which is only true is there is no caching and I highly doubt that these files are configured not to be cached after the first load