First of all let us understand what is Expansion Ratio. Expansion Ratio is the ratio of quantity of foam generated to the quantity of foam used. Simply speaking it gives an idea of the amount of foam that can be generated by using unit quantity of foam.
Therefore, Expansion Ratio = (Quantity of foam generated) / (Quantity of foam solution used)

Low expansion foams are those solution which expands less to generate foam and therefore they have lesser expansion ratio of about 12:1. Low expansion foams are heavier and therefore used on open deck fires as it is not carried away by the wind easily.
High expansion foams on the other hand, is lighter compared to low expansion foam and has a expansion ratio of above 100:1 and therefore expands more. This foam is mainly used in fighting fire in confined spaces such as Engine Room. It is not used in outdoor fires as it is easily carried away by the wind.