Is 125 grams equal to 100 mls?

It depends on the substance being measured. If it's water, 1g = 1ml so 125g = 125ml.

The best way to get round it would be to place your measuring jug on your scales, adjust them so they read 0g while the jug is in place and then pour whatever it is you're trying to measure into the jog until 125g of it is reached.