Redshift DISTSTYLE ALL table takes less disk space?
Well, after lots of experimentation, we have found that, yes, if you have a fat table with not enough rows to fill much of a block at the slice level using DISTSTYLE KEY, it is much more efficient, space-wise, to use DISTSTYLE ALL, even though this replicates more data.

For example, our fat tables have, say, 600000 rows and 1000 columns. Then, on a 5-node 32-slice setup, they were using up 1Mb per column per node per slice, or 160 Gb, and we were only filling less than 1% of that space.

DISTSTYLE ALL appears to not use all the slices.

So using DISTSTYLE ALL, we multiplied the data by 5, but this still takes up less than 1/20th of the space that the DISTSTYLE KEY tables did.

It slows down our queries a little (due to the need for IPC between the slices, and because the DISTSTYLE KEY means we have only the data on the node that matches our other data) but, since these tables are loaded once and never change after, we don't suffer from the other problems of DISTSTYLE ALL.