shapes.txt exists so a map can draw where the bus goes. A line on a phone screen does not need a point every few centimetres, but some feeds export straight from survey-grade GIS and ship every vertex anyway. The result is a shapes.txt bigger than the rest of the feed combined, downloaded by every consumer, every day, to draw the same line.
The law
The median distance between consecutive points of a shape is under 2 m, and shapes.txt is the largest file in the zip.
How to get pardoned
Simplify the shapes before you publish them. Douglas-Peucker with a tolerance of a few metres keeps every curve a map can show and removes the rest.