rabbitoid: what makes such a difference to regular chess?

The contribution from the extra squares is not too bad, but in Gothic Chess we have 7 piece types other than the king, whereas chess only has 5. So, in order to solve a "complete set", we have more permutations per database.

With 7 pieces x 2 sides to move, we have 14 database slices to solve to get the 3-piece database.

This grows to (7 x (7 - 1)) x 2 = 84 for the 4-piece database set. The "7 - 1" reflects that there are some databases that are their own "colors reversed" database, suck as King + Queen vs. King + Queen, King + Chancellor vs. King + Chancellor, etc. A black to move position can be looked up even if we solve the database with only having white to move.

Compunding this with the long mating distances, such as some of the ones shown at http://www.gothicchess.org/databases.html and now we need to use 2 bytes per position, instead of one byte per position.

So we have more positions per slice, more slices, and need more storage per position.