i've just looked in i386 machdep code:
avail memory=total memory-buffer cache size-kernel size-all kernel data
size.

so (with 15MB memory available):

Feb 17 01:42:46 wojtek /netbsd: NetBSD 1.5 (local) #4: Fri Feb 16 12:03:13 CET 2001
Feb 17 01:42:46 wojtek /netbsd:     wojtek@wojtek.from.pl:/usr/src/sys/arch/i386/compile/local
Feb 17 01:42:46 wojtek /netbsd: cpu0: Intel Pentium/MMX (586-class)
Feb 17 01:42:46 wojtek /netbsd: total memory = 14976 KB
Feb 17 01:42:46 wojtek /netbsd: avail memory = 12324 KB
Feb 17 01:42:46 wojtek /netbsd: using 212 buffers containing 848 KB of memory

14976=all right it's 15*1024-384, as 384 is not available on i386 (between
640 and 1M)

12324=14976-848-1372-x

1372=kernel size.

x=432kB

so kernel needs 432kB for it's own data, quite large but i think it's OK

the problem is that after adding swap, running some programs no more than
8MB (when summing vmstat -sm data) is available instead of 12.

with 8MB case, i have maximum of 1.5MB available and constant swapping!

where is an error?