I have a surprisingly small and light Probus Antoninian. It is the coin, first row on the right with the consular bust. The coins were photographed together, so they show the correct relative size. Measurements: The coin on the left: 3.93 g, 22mm The coin on the right: 2.45g, 19mm I have chosen the coin on the right, because it has a normal sized flan without excess metal and because it has the same reverse image. Coin on the left: legend is unbroken, the temple is quite big Coin on the right: the legend is broken and shortened and the temple is squeezed together. Here is a comparison of the obverse design with the same design from the same mint. Coin on the left: large bust, long sceptre Coin on the right: small bust, short sceptre Is it possible, that the coin on the right was not intended as an Antoninianus (despite the radiate crown) but as a Denarius? Or are those differences within the normal range?
What are the die diameters? They look to be fairly close to me, in which case I'd say it's just an accidental flan size difference. Die diameter is better guide to denomination than flan size or weight. (BTW, your lefts and rights aren't always consistent, needs some editing. ) Here's a Carinus denarius, 18mm and 2.09g, the die diameter is only 16mm though.
I have about 40 Probus Antoniniae, but this one stands out as being the lightest and the smallest in absolute size and in die diameter. Also I compared the two consular busts above. The normal bust is 17mm, the bust on the small coin is 14mm in hight. The sceptre on the large coin is 7mm, but only 3mm on the small coin. I would argue that the die was made for a smaller than usual flan. If this was an experiment to reduce the size and weight of the Antoninian, or whether the coin was intended to circulate as a Denarius or whether this is just a freak accident, I don't know. However, the only other rather small Antoninian of Probus is this one below. It is from an eastern mint (unidentified 4th eastern mint). The diameter is 19mm-21mm. The die diameter is 18mm, which is the same as the small coin from Rome
The buying power of the antoninianus/aurelianus was totally decoupled from any intrinsic value, so output capacity over time was more important than maintaining tight standards. Expecting the population to be able to keep track of two separate denominations based only on very slight differences in diameter would have been a disaster, like the British double florin which was easily mistaken for the more common crown.
Then again, we know that both double-denari (Antoniniae) and denari were issued under Probus. The single denari were distinguishable by a laurel wreath and smaller measurements. The problem is that the small coin above shows a radiate crown (typical for an Antoninian) with measurements that are more in line with a denarius.
Time to accumulate a whole bunch of data on Aurelian to Carinus antoninianus measurement! I tend to think you're right that the die was designed for a smaller flan, but agree with @Finn235 that this was likely due to a shortage of metal rather than a different denomination. Here's an example of a puzzler that surely is a different denomination: It is 1.86g and only 13mm. That's quinarius size, but the radiate crown doesn't fit. Looks like the die diameter might be about 14mm but I don't know for sure, because sadly enough, the coin never arrived from Germany.
PROBUS Antoninianus OBVERSE: IMP PROBVS AVG, radiate mantled bust left holding eagle-tipped sceptre REVERSE: SOLI INVICTO, Sol in galloping quadriga left, R-thunderbolt-B in ex. Struck at Rome, 275-6 AD 4.2g, 24mm RIC 202
