So the full-size pattern works by bouncing a signal back-and-forth between the construction site and an oncoming GPSE. Each time it does this it produces either one or two gliders depending on the positioning of the GPSE. It is these gliders that do the construction.
But the 'recipe' contains 1665791 bits, meaning the signal has to bounce back-and-forth 1665791 times. Because the distance to the GPSE halves every time, it would have to start at a distance of 2^1665791, which is impractically large.
So instead we only have it bounce back-and-forth a small number of times (26), and insert the other 1665765 bits 'manually' by adding streams of 1665765 spaceships that collide near the construction site.
But the 'recipe' contains 1665791 bits, meaning the signal has to bounce back-and-forth 1665791 times. Because the distance to the GPSE halves every time, it would have to start at a distance of 2^1665791, which is impractically large.
So instead we only have it bounce back-and-forth a small number of times (26), and insert the other 1665765 bits 'manually' by adding streams of 1665765 spaceships that collide near the construction site.