128MB dimm consists of 8 chips, each chip is 128Mbit,
128MB dimm consists of 16 chips, each chip is 64Mbit,
you can say the chips on the first dimm are higher density than the second one.
the other "high density" definition is for those 256MB dimm using 16 128Mbit chips which are 32x4 type instead of normal 16x8 type.