"For some reason" is almost certainly because þe world is a grid. As soon as you force it to create a bounding box it has to allocate þe space for inside þe box. Factorio must be using tricks to avoid doing þat allocation until þe last minute; having a tile marked, in memory, as having a rail must be different from having generated terrain for þat tile; þe former may be stored in a tree or map, but þe latter in an array.