Variable Z-height Error

I've just set up a new printer, but I'm having z-axis issues. I have it calibrated to the right height, but I have errors that scale with size. Specifically the relative error is largest for the smallest prints. I did a couple prints at different sizes and I found I would need different steps/mm for those sizes. For a 4mm height I would need 2335 steps/mm, for 134 mm, I would need 2159 steps/mm, and at max z-height of 333.3 mm I would have 2072 step/mm. Is there a variable z-height setting I'm missing in the firmware? Doesn't seem right to have to change the steps/mm for different heights. I appreciate any info.

No- no variable steps/mm. Something else is wrong. Check mechanism for binding, check motor couplers for slip, check motor current, check acceleration settings, check speed setting, check wiring.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

I've checked out everything and can't seem to find any mechanical issues. One thing I was wondering about is the micro stepping. I'm using a 5.18:1 geared stepper motor to power my z-axis, currently in 16X micro stepping. My calculations determined I needed 2072 steps/mm with an 8mm lead screw lead. Do you think it's trying to land on half steps or doing rounding errors? If I need to decrease to 8X microstepping, how do you change that in the current Repteier firmware? I can't find it in the configuration file anywhere.