Microstrip blocking capacitor ground cutout -- large cutout size
Does this result seem reasonable?


It is reasonable as a simulation result. 

By cutting out the ground plane, you force the return currents to flow further from the signal currents. This increases the inductance of the structure. With sufficient inductance and the lumped capacitor you form an LC circuit that at some frequency looks like a short between one section of transmission line and the the other. Using your optimization procedure you adjusted the geometry to make this frequency equal to your operating frequency.

Practically, it might not be a good idea to try to use this effect to match your line because


The resonant frequency will be affected by manufacturing variation of the dielectric thickness, dielectric constant, copper thickness, capacitor value, etc. 
There is some uncertainty about the capacitor model. You haven't said what model you use (lumped capacitance, RLC equivalent circuit, 2-port s-parameters, ...) but very likely the model provided by the manufacturer includes some effect from the test fixture they mounted it in during characterization. How to back-out those fixture effects to predict the behavior in your circuit geometry is a tricky question.


As mentioned by Ale..chenski in comments, we more often use ground plane cut-outs for pad matching when the pad width is greater than the transmission line width we're trying to match.