Say you'd used 0.000000001 ohm resistors - would you expect that to work too? I hope not...
You were simply overloading your amp, whatever that was, and greatly reducing the amplitude to a few LSBs of the ADC, giving a steppy output.
Ironically you didn't post any details of the hardware so we'd not have been able to figure things out for you (that's why posting everything (code, hardware details, circuit, photo) is a great idea if you want a quick solution - many eyes over everything usually spot the problem).