A 300-400 person A380 is a very light A380: Emirates go from 489 passengers (ultra long-range version) up to 615 passengers (2 class long-range). The A380 is that huge. Also I don't think there are many 100 passengers airliners capable of crossing the Atlantic: long range single aisle (A321NEO-LR) seats around 180.
Look at BA flight 001, which is a transatlantic flight from LCY/SNN to JFK in an Airbus A318-100 [0] with only 32 (business class) seats. According to BA, a standard two class configuration would seat 107. Admittedly, it has to refuel in Shannon to allow it to make the Westbound leg, but the Eastbound trip goes non-stop from New York to London.
On the other hand the A318 is not in production anymore, and I doubt it would cross the pond with 107 seats. There's two tendencies in the market of airliners at the moment: single aisles are getting bigger, and twin aisles are getting smaller.
This is pretty noticeable on Airbus side for single aisles: the A321 used to be little more than an anecdote, now Airbus is ramping up the A321NEO to be about a third of its entire A320NEO family production (anticipating upsizing in the order book).
On the wide-body side many 747 have been replaced by 777-300ER, A350-900 and 787-9. Quite the downsizing. 787-9, A350-900 and 787-10 are raking in orders. Meanwhile the A380 is dying, the 747-8 is all but dead (though the 747-8F is still there). The 777-9 order book is a caricature of that of the A380 (Emirates + some anecdotes, even more so with Etihad likely to cancel). A350-1000 and 777-8 sales are underwhelming to say the least (although the latter will become a tremendous freighter).
