It's a common thing but let's analyze generally (also note it comes with easy starts and other weight-gaining stuff) :
2511- lightest
310- lightest
352- lightest
370- well, sorry. At least good feature package and relatively huge stroke and outcoming torque
420 (400 in US) yep, sorry again though it's survivable 200grams from H440
390-very light, long stroke, it's unavailable in US
4510- lightest per power
490- lightest 50cc before 501 was released
501- lightest 50cc
590- decent weight if you compare with what it was made to compete with
620- lighter with both dogs than now 562xp with recent aluminum crankcase which probably stopped falling apart
And so on...
Basically this all "very heavy" comes mostly from certain guy who once upon a time heard somewhere in northern country that ECHO produce less power (= less torque on ~10k RPMs) without having a clue how they cut with higher load and after run-in. Don't get caught - it's generally misleading