I agree. He also has really small hands. My hands are bigger than his and I am not six feet tall. He's 6'3" with 9" hands (measured from thumb to little finger outstretched).
Hand size in certainly a consideration in evaluating draft talent, and goes some ways toward accounting for a player's ability to catch well or not, as the case may be. But I think that consideration fades into the background once a guy has a couple of years in the league.
For example, Golden Tate's Combine measurement was 9 1/4" albeit in a much smaller frame, and he arguably has the best hands in the NFL. Antonio Brown, another 5'10" guy, has 9" hands. There are plenty of other smaller frame receivers with small hands. And I don't think a 1" difference between Janis and Nelson's 10" hands, to compare two big body guys, comes near accounting for the notable difference.
Demonstration of soft hands takes precedence over any size limitation.
There's a secret sauce that goes into it...a holistic combination of eye/hand coordination, body control in the sense of the arms and hands being in sync with the other body movements at the point of the catch, acute depth perception, fearlessness, competitiveness in the moment...and any number of other factors one wants to throw into the basket.
I firmly believe McCarthy is correct in not trying to break it down into components when his eye test evaluates a guy as "a natural catcher of the football", or not as the case may be, a phrase he's used positively with Abbrederis or with Hyde fielding punts, or negatively in the case of Janis as recently as this week according to Gannon.
You can emphasize focus; you can put him in front of a jugs machine and have him hand catch a 100 balls per day. That can help, but ultimately it comes down to the secret sauce.
One question I have with Janis is whether his body catching was a bad habit acquired early on with undemanding coaching or whether it was an adaptation to having innately hard hands. I'm willing to wait a little longer, but evidence is mounting that hard hands are a deficiency that will have to be taken into account.