Hagen Poiseuille equation will tell you the principles. I've tried to fit your numbers to the equation, but maths isn't my strong point. An abbreviated version:
Flow = Pi x pressure gradient x radius ^4
--------------------------------------------
Constant x viscosity x length tube
Flow varies proportionally to Pressure gradient along the tube, and to the radius of the tube raised to the power of 4. More pressure or bigger tube = more flow
It varies inversely to length of tube and viscosity of the fluid. Long tubes and viscous fluids will restrict flow.
So radius has a big effect, due to it being raised to the power of 4.
The maths in your case is the bit I'm not sure about. If diameter is 16mm radius = 8mm. I'll save you some boring bits but comparing 8^4 = 4096 vs 9.5^4 = 8145
The ratio is ~ half, so if all other factors remain the same then flow should also be halved. But I'll admit that seems a big drop in flow for loosing 3mm off a 19mm diameter.
Also, Surface area of 16mm pipe 200mm2 vs 280mm2 for 19mm, by my calculation. This doesn't relate to the HP equation but will give you a rough idea that internal resistance will increase and reduce flow.
Hopefully someone with better maths will come and clarify.
Overall, I'd not be suprised if you lost at least a third of the flow, although my fag packet maths suggests half.
If you must use 16mm pipe, you can mitigate the flow drop to some extent by:
1. reducing the length of the tube to a minimum
2. maximising the pressure along the tube by raising the filter up and reducing losses due to water head pressure.