This is a really neat denomination. I remember reading that Caracalla's introduction of the ant led to some inflation because the "nominal 2x denarius" value was only more like 1.3-1.5x silver content. Although it's larger, I somehow doubt this double ant had double the silver. Or did it?
It's no larger or heavier than the antoninianus, but it did in fact have double the silver. This page by @dougsmit discusses the XXI mark representing "20 parts copper and 1 part of silver." And regarding the double antoniniani, the second coin in the OP was part of a group used in a metallurgical study published by @Valentinian that established that the XI coins of Tacitus did in fact have twice the silver content of the regular antoninianus.
I just went back to read your OP, @zumbly, and saw the provenance. I can happily provide you with some more info on that. The "Maple Leaf" collector is none other than our own @Terence Cheesman. Here's my own ex Cheesman coin, a post-reform radiate of Constantius I (Heraclea).
Good to know! I picked up a few from that sale that I really liked, but the Tacitus is my favorite. I'll enjoy them even better now knowing the collector behind the Maple Leaf monicker.
Man, that is a beauty! A nice Gratian and lovely little green Thessalonica-only Valentinian II campgate... GRATIAN AE3. 2.74g, 19.2mm. Antioch mint, AD 367. NOT IN RIC, cf. RIC IX 36c.1. O: D N GRATI-ANVS AVG, diademed, draped and cuirassed bust right. R: SECVRITAS-REIPVBLICAE, Victory advancing left, holding wreath and palm; C in left field, ANTI in exergue. Ex Maple Leaf Collection VALENTINIAN II AE4. 0.79g, 12mm. Thessalonica mint, AD 384-388. RIC IX 62a.2; LRBC 1864. O: D N VALENTIN[IANVS P F AVG], pearl-diademed, draped and cuirassed bust right. R: GLORIA REI-[PVB]LCE, campgate with four layers, no doors and two turrets; A in left field, TES in exergue. Ex Maple Leaf Collection Finally, this nice not-in-RIC Licinius I... LICINIUS I AE Follis. 5.91g, 26.5mm. Cyzicus mint, AD 308/9. NOT IN RIC, cf. RIC VI 42 (for Galerius). O: VAL LICINNIANVS LICINNIVS P F AVG, laureate head right. R: GENIO A-VGVSTI, Genius standing facing, head left, holding patera and cornucopiae; A in left field; MKV in exergue. Ex Maple Leaf Collection
Entry 92.3 from Mr. Esty's metallurgical study: From an Israeli collector, approved by the Israel Antiquities Authority: From a Dutch seller, dispatching a collection gathered in the Middle East:
Those are great. The huge C in the field on the Antioch Gratian is wild - is that the "not-in-RIC" aspect? And surely your Licinius and Doug's came from the same engraver's hand... Looks like Terence kept Triskeles stocked for quite a few auctions!