They're both 5'11", but Celebrini is listed at 181 lbs, Eiserman 192 lbs. I agree they're similar size, but Eiserman is certainly stronger, more physically developed.
However, I think the bigger thing is skillset - is Eiserman closer to his ceiling than Celebrini? When taking a forward 1st overall, you want a guy who is really dynamic, a star who can create offence out of nothing and break down defences on his own. Celebrini is that - the speed, the hands, the elusiveness, the offensive creativity, he's so dynamic, his game screams "star." Eiserman is a great skater too, but doesn't have the pure speed or deceptiveness of Celebrini. He's a bit more of a north/south player with an elite shot, strong on the puck, nice puck protection, very polished/NHL ready game. The eye test for Eiserman is "this guy looks like a man amongst boys, borderline NHL ready today", but he doesn't necessarily wow you with dynamic moves. Celebrini wows you.
Eiserman is terrific, but does he have the same pure upside as Celebrini? My gut says no, personally, but I could be wrong (I often am). IMO when ppl say "Eiserman might be an early developer", they mean both physically, and in terms of polish to his game, with not as dynamic a play style as you might like in a 1OA forward. Sometimes these types turn out - for example Tavares became a franchise C with this profile, Hischier is becoming very good. But there's also guys like Slaf, Shane Wright, Laff, Kakko, who are all certainly very young with uncertain futures (could become stars!), but there's some concerns they were early developers, who were ahead of their peers in physicality/polish/maturity, but won't be NHL stars.