Prusa XI3 Extruder Calibration
It is really strange that although you increased the steps per mm, the amount extruded was less. I can think of two possible explanations:


You are extruding too quickly, at a rate at which the extruder can't keep up melting the filament fast enough, causing the filament to slip or the extruder to miss steps: try lowering the feedrate (a feedrate like 100mm/min is typical for 1.75mm filament) and make sure that the temperature is appropriate to your filament.
You are in absolute coordinate mode, and when you try to extrude 100mm it actually extrudes a different amount (based on the previous "position" of the extruder). Enter relative coordinate mode using G91.