Throat size is one element in determining the resistance of the system (the others being instrument bore size, mouthpiece gap and embouchure aperture). Its possible to compensate against a very resistive instrument by using a mouthpiece with a larger throat (bore) size or against a very open instrument by using a tighter mouthpiece.
Bach
Standard - #27, 3.66mm
Megatone - #26, 3.73mm (larger throat designed to overcome the tight feeling that results from the larger mass of the megatone mouthpiece)
Bach used to (maybe still) do custom mouthpiece work and I have seen a 5V (very deep V cup) with a larger #20 throat that was stamped "20" in the factory.
Schilke
Standard line - #26 (except models with "A" backbore which are #27)
Schilke now make deep V shaped "symphony" models. I do not know what throat sizes these use.
Denis Wick
Models without letters - 4.6mm #15
except: 2 which is #14
B models - 4.3mm #18
S model - 4.1mm #20
These mouthpieces were originally designed for the old Imperial and early Sovereign cornets which had more resistance than current models. I feel that they tend to encourage over blowing in modern, freer blowing, instruments. Wick seem to have recognise this issue by making a 4 ½ which is identical to the 4 but with a narrower #16 throat.
Curry
BBC (Brass Band Cornet) - 4.07mm #20
VC (Vintage Cornet) - #18
DC (Deep Cornet) - #25
Yamaha (short shank)
11E4 - 4.2mm #20
16E & 14E - 3.98mm #22
David King - 4.60mm #15 (presumably influenced by the Wick standard of larger throats)
Warburton
Long shank (standard) #27
(Warburton make #23 mouthpieces as a standard option so these are also in circulation but marked "23")
Short shank with "BC" tops for UK market #20 (unconfirmed though as I have not seen one personally)
Other factors
The shape of the cup of a mouthpiece also affects resistance with flatter, shallower cups generally being more resistive. You can test the relative resistances of two mouthpieces by running a constant stream of water into them from a tap and observing how much it will take before the cup starts to overflow. Its surprising how much some apparently small mouthpieces will take (try comparing a smaller Jet-Tone trumpet mouthpiece with a standard Bach 7c - the apparently small Jet-Tone will handle as much flow).
