A Bach 5C is nowhere near 16.25mm. Do not follow the published Bach measurements. They are not accurate and do not state where the diameter is measured. Laskey, Curry, and Schilke, just to name a few measure, the 5C at .660" or 16.76 mm.
A quick look at the Kanstul comparator shows a 5C to have more cup volume than a 1 1/2 C and a much more rounded rim.
The Bach 5C is for players who do not like a sharp bite on the rim and it requires well developed chops to play it. It is not made for underdeveloped embouchures.
I used both a 5C and a Reeves 42 and they are really are very different mouthpieces. A Curry 5M I used for a while had a very similar feel to a Bach 5C but did not play like the 5C. The Curry was more efficient. The Reeves felt much smaller to both of them.
Follow Rowuk's advice. It is spot on and concurs with what a mouthpiece manufacture will tell you.