Rozanne is correct - when you boil the juice it thickens up, as moisture evaporates from it. Keep it boiling until the weight reduces by half. Ski, this is what I did:
I weighed a pan empty and noted down the weight - 1250g.
Added whatever passion fruit juice I had and weighed again - 1495g. I think I had juice from about 16 PF.
Difference is 1495 - 1250 = 245g of juice.
To reduce to 50% weight divide 245 by 2 = 120g approx
Add to the weight of the pot 1250 + 120 = 1370g
Boil away weighing every few minutes until the pot with the puree weighs 1370g
Voila - 120g passion fruit puree!
I do the same with raspberry juice to make puree.