Stepper: Distribution of Microsteps within a full step
The assumptions in your question are correct. In microstepping mode, each step is divided into a number of smaller steps. Hence, 8 microsteps mean, that a full step is divided into 8 equally long smaller steps. The limit is the mechanics. Up to 16 microsteps, you can expect the microsteps to be equidistant. After that, the mechanical deviations of the motor result in the steps being of slightly different size. 

I advice you to have a look into the data sheets of Allegro Microsystems A3977 or A3979 stepper motor drivers. They have a rather detailed and easy to understand description of how microstepping works. 

The pictures you mention in the comments indicate, that there is a problem with the driver. Measure the current of the motor coils. That is done by inserting a resistor and measuring the voltage across this resistor, not coil to GND. If you do that for both coils simultaneously, be aware that the GND of the probes make a shortcut. So, use DIFFERENTIAL probes. For 4/8/16 microsteps (only half-step is not enough, better use 1/16-steps mode as you intend straightaway), you should see a screen like "Figure 16. Microstepping Drive Current" in the data sheet. If this is not the case, there is a problem with the driver. As long as this is not clear, it is rather useless to guess about possible further sources of error. 

The current through the two motor windings have 90° phase shift. This is absolutely essential. This is the reason, why you should check both windings. Unfortunately, the DRV8711 has no HOME-output, which is useful as a trigger for measuring the motor currents separately. If you can, get at least one differential probe and measure the currents simultaneously. 

It is also essential that the windings are connected in the same polarity. The data sheet of the motor gives you details of how to wire the motor properly.

Last personal hint: Nanotec is crap! I have never met anybody in more than 30 years in electronics now, who spoke in favor of Nanotec. Having just checked the web-site of Nanotec, this was confirmed in the usual unpleasant way, Nanotec is known for.  Real stepper motors are made by Oriental Motors or SANYO DENKI. Use one of these. Oriental optimizes their steppers especially for microstepping.