Pīpipi vs Yellowhead

The yellowhead is heavier: 27.7 g against 12.1 g for the pīpipi, about 2.3 times the weight.

What is the difference between a pīpipi and a yellowhead?

Each point below is worked out from a pair of recorded values.

The yellowhead is heavier: 27.7 g against 12.1 g for the pīpipi, about 2.3 times the weight.
Both belong to the genus Mohoua, so they are close relatives.
The pīpipi is listed as Least Concern on the IUCN Red List; the yellowhead is Near Threatened.
They are on record for 1 of the same countries (the pīpipi for 1 in total, the yellowhead for 1). Country data is incomplete and can include zoos, so this does not show whether they meet in the wild.

A scale drawing needs a recorded length for both animals; only weights are on record for this pair.