Today I did a quick down and dirty comparison (hear recording below) of my current narrow tube setup and a large tube setup, both similar to the setups pictured above. The large tubing seemed to be latex rubber.
If your playback software can display the sound clip, you can jump back and forth between the narrow tube and large tube sections of the recording to hear the difference.
To my ears, I did not hear any consistent difference between the narrow and large diameter tubing. I was surprised by this because I definitely heard a difference when I tried this comparison a number of weeks ago with a slightly different set of conditions.
There are too many variables I am not accounting for in this quick experiment: melodica type, tubing construction, and mouthpiece design to name a few.
I have not yet used the large diameter tubing I thought would be best because I cannot connect it easily to the fitting. I will be trying this and additional explorations in the effects tubing diameter on sound quality as the weeks go by and will report my findings.
I looked for the recording from my previous test but have not found it yet.