I'm not usually one for a harsh critique, but there will be a MASSIVE quality drop on these heads when reduced to ingame size. Let me illustrate.
128x128 resolution leaves you with a total of 16,384 pixels.Doing your head in 512x512 will result in 262,144 pixels.
That's 16 times the original amount! Therefore, any lines that are less than 16 pixels wide will be reduced to a blur of (in your case) black and green pixels to help compensate for the fact that you can't have a <1 pixel wide line.
Doing heads in 512 without actively thinking about the quality loss while doing them often results in very crappy ingame heads. My advice would be to stick to a MAX of 256x256.