uncle tom wrote: ↑09:55 Fri 14 May 2021
Before you toss anyone under the bus, there's a legitimate reason. Port, being roughly 20% ABV, requires about a 1/3 more headspace for expansion than does a still wine at, say, 12% ABV. Very large bottles like that can also have some slight internal variations. So they are filled by actual liquid measurement and not by level into the neck. So some may be slightly higher or lower as a result.
Sorry Andy, I don't know who gave you that explanation, but it's nonsense. I've got loads of VP in varying formats, and plenty with fills that I classify as IN++ - which means a level within 3/8" (10mm) from the cork. Mags and double mags often have excellent levels.
I suspect it's all a question of risk, considering the temperature changes you expect the bottle to be exposed to.
Interesting snippet from S. Barry's paper on "Temperature driven air flow through wine corks":
The typical wine expansion coefficient is β = 2 × 10−4/K [2, 6, 7]. For a
standard 750 ml bottle of wine the volume increase over 30 degrees is reported
to be in the range of 4.73–7.4 ml compared with a head space volume in the
order of 5–15 ml. Hence the upper limit of proportional mass loss due to wine
expansion is of the order of 30–100% and is hence very comparable to the
changes due to pressure increases within the head space. This also implies
that overfilled bottles, with little headspace, are particularly prone to wine
expansion effects if temperate changes are large. Anecdotally, manufacturers
have reported international shipments of wine where temperature changes
have been large enough for the pressure to expel a third of the corks. Almost
certainly this is due to insufficient headspace to allow for wine expansion.
While I can't speak to Andy's statement regarding the different expansion for the higher ABV, the above would at least suggest that if the neck were proportionally wider in the larger formats, then the same fill level would be acceptable; however typically many larger formats (especially magnums) have a higher main-bottle width to neck width ratio, suggesting the acceptable max-fill level for the same pressure risk would be lower.
Therefore, some quick maths:
Measuring a random 750ml port bottle, the bottle:neck ratio is roughly 8:3
Measuring a random 1500ml port magnum, the bottle:neck ratio is roughly 11:4
If the max fill level for acceptable risk on a standard 750ml port bottle is 10mm below cork, then on the magnum it would be 10*(11/4)/(8/3) = 10.3ml
I don't have any larger formats to hand to measure, but the increase/risk equation seems to suggest that unless the bottle:neck ratio is substantially higher then the gap need not be significantly taller for the same risk level during transport. As to the initial acceptable risk, that I cannot substantiate; the above would suggest it perhaps unwise to leave less than 10ml of space in a 750ml wine bottle, possible adjustment needed for higher ABV (unknown), and less needed if you're certain it won't be exposed to significant temperature changes.