That actually is true, as the 543xp is 43cc, not 42. It is a pro saw though, not a clamshell.
The 543xp is both lighter and no doubt more nimble than th0 420/421 and has the same power rating. It isn't a true Husqvarna though, but a Zenoah.
There have been very few reports on how those saws run though, a I haven't seen a single real comparison vs. the MS241 and/or the PS-421....
Personally, I'm not really interested in any of these 42/43cc saws (or the Echo 500/490), as the 550xp (or 346xp) isn't much heavier, and the power difference is huge.