10 years ago
I know Facehuggers can latch on to almost anything, but that clearly wasn't a Facehugger.
It's heavily implied (with evidence, even) that it was a chestburster that emerged from the Engineer in Alien. Strangely, his dead Facehugger was nowhere to be found, if I'm recalling that right. In other words, his chest was the only affected area. The Deacon, however, split the entire goddamn body.
I was going to make an argument about how Deacons and Xenos even emerge differently from their hosts, but the Dogalien in Alien 3 made a similar entrance in both cuts of the movie. It's unexplained in-universe why the Dogalien came out so big, but one possible explanation for the Oxalien doing the same was that it simply couldn't get out until it was further along in its developed. This might be good for the Dogalien too, I don't remember how big its chestburster was compared to the dog.
Whether or not this is also plausible for the Deacon is up to you. I personally don't think so - the Dogalien had to struggle to to get out while the Deacon came out smoothly and naturally.