They do have deceptively cavernous mouths! I had one once and assumed the other fish in the tank were too large to be eaten because it was quite small. I was incorrect and had to move the remaining fish to a new tank.
Very deceptive - and they're not above trying to fit things they think just might fit. One of the local fish stores had a particularly large specimen awhile back, and had it housed with a large pleco and some large Metynnis. While the Metynnis were large enough to not be eaten, that didn't stop the arowana from partially engulfing one of them and half-mauling it in the process. As the arowana continued its interest in attempting to eat the Metynnis they had to be promptly removed.