The answer is 24MB.
… and that’s for an initial full install weighting 63MB. The reason for it being small (other than the fact that you \*can\* do such a full update) is that IPS, the packaging system behind GlassFish v3 works at the file level rather than with package granularity.
Your mileage may vary depending on the jump you’re trying to make (in my case this was from b47 to b49).
Here’s a quick replay of my experience :
% bin/pkg set-authority -P -O http://pkg.glassfish.org/v3/dev --enable dev.glassfish.org
% bin/pkg image-update
DOWNLOAD PKGS FILES XFER (MB)
Completed 32/32 198/198 24.10/24.10
Removal Phase 64/64
Install Phase 39/39
Update Phase 193/193
Reading Existing Index 9/9
Indexing Packages 41/41