Portions/charges of liquids are nowhere linked to a specific mass or volume. 10 units of gasoline might be as heavy as 1000 units of water, or not. Mass and volume are listed in addition to charges.
Charges/portions are stored as an integer, therefor each consumption must use at least a single unit of it (it can’t use 0.5 units, the game cant store the remaining 0.5 units).
If for example alcohols like whiskey would be measured in 1 unit = 250ml (like water) you had to drink at least 250ml every time you consume it. Similar with gasoline consummation in cars. If water is implemented as 25ml = 7 portions (like alcohols) you had to do the drinking action 7 times as often because each time it consumes only one charge.