From Wikitech

TIFFs has 2 custom engines. The vips engine is leveraged when the TIFF is over a size limit, defined by the VIPS_ENGINE_MIN_PIXELS Thumbor configuration value. Otherwise the tiff engine is used (which is really just a thin wrapper for our default custom Imagemagick engine).

Both engines handle multipage support via the "page" filter. If the requested page fails (eg. is out of bounds), we retry using the first page of the document.

We determine the size of the image that will select engine or the other via exiftool, running inside the vips engine. Note that this is a bit of an oversimplified approach, as in a multipage situation, this would look at the size of the first page of the document and not the page actually being requested.