According to f1analisitecnica.com, the fact that the car is circa 5kgs heavier is not down to the long wheelbase.
1st paragraph Floor
The floor used in Barcellona tests and Australia couldn't give exact data to be studied (so work has been dampered). The one introduced in China couldn't do miracles.
2nd paragraph Weight
Mainly it's because of an oversized rear suspension, the engineers were too cautious when guessing the G forces expected for this season (on a sidenote, Merc were said/did say they expected to hit380+ km/h at Monza in 2014?).
3rd paragraph
In order to simulate a ligher rear of the car, they took off T-wing and wastegate pipes
http://www.f1analisitecnica.com/2017/04 ... ml?refresh