I can't offer you much of a "canonical" answer for your perceived inconsistency, but I would posit that "canon" in the Alien films is a bit of an odd...uh...beast anyway. As to Ridley Scott there isn't much of canon other than his 3 films and even if we decide to view all the Alien films and the Predator films as sharing a canonical universe, then it easily becomes a bit littered with incongruencies that you somewhat have to accept as different interpretations by different film-makers.
That being said, I don't think there actually is much of an inconsistency here. We really only have one other instance of a somewhat closed life cycle we can compare it to, and that is the Nostromo Alien from the eponymous film. In all other films we don't really get to follow individual aliens around and for all we know, the "adults" we encounter could be days old, or just hours if not minutes.
So let's look at the first film, where the alien also grows really fast. If you remember it gets given "birth" by Kane and quickly runs away. The crew then goes searching for it. It's not quite clear after how long, Ash built some kind of motion tracker meanwhile (if he didn't already have it ready, being rather ahead of the matter all the time). But it's likely not more than a few hours if even that. You don't want this thing run around too long on your ship.
But the point is, even the crew still thought it was just the size of a cat like when it slipped out of Kane. They find its skin, hinting at a rather quick growth process and later when it kills its first victim Brett, Parker realizes that it's the size of a big man and is rather surprised at that.
The son of a bitch is huge! I mean, it's like a man. It's big!
So to reiterate, we don't really know how long it took to grow that big, but it's probably not longer than an hour and nothing says it didn't just happen in 5 minutes after the alien ran away. It sure grows really quickly. I would agree it all seems rather fast in Alien: Covenant, but not unusual in comparison to Alien, and also quite in line with the theme of a "perfect organism" that the films often try to evoke.
Add to this, that for all we know the Covenant alien might still be a somewhat different strand of evolution than the one later encountered by the Nostromo, even though it already is David's final creation and comes very close to the alien we know and
love fear. But I think it still looks a bit different and generally "bulkier" than the one from the first film. (However, that again might still be down to just different filmic interpretations.)
But as to how it can grow so quickly, I neither am able to answer this from a xenobiological standpoint, nor do I think it really makes sense to reason about the Alien lifecycle's actual plausibility (if that is even considered on-topic). For all the thought that is put into these creatures and their supposed perfection, it still is rather made-up and doesn't always make perfect sense nor has to adhere to all too narrow biological restrictions. For example it's even "worse" than you think when you say "the first thing it eats ostensibly are the humans", because I think the bodies they find in Alien: Covenant are surprisingly intact and the aliens seem to deliberately not eat their hosts (like Oram or Lope) or even the people they kill (like Rosenthal). We really don't know how they actually function, but I don't think that's much of a problem. Add to this again the different interpreations by different film-makers that introduce their own inconsistencies (e.g. in the first Alien film it was thought to turn humans into eggs, while later films added the concept of a queen).