Just going to correct you there Anthony; for class B fire ratings i.e. 144B relates to the total volume of 'liquid' not heptane. The test calls for 144 litres and that should be made up of 1/3 water and 2/3 heptane. The heptane floats on top of the water and the water is used to keep the test tray cool. So in fact a 144B test fire only has 96 litres of heptane in it.
For clarity, when talking about class A fire ratings, the rating on an extinguisher (13A, 21A, 55A etc) relates to the length in metres of the test fire (crib). A 13A relates to 1.3 metres in length, 21A is 2.1metres, 55A is 5.5 metres.
Anthony is correct that there are 3 litre foam extinguishers that come with high performance foam concentrate and can have a 13A rating and so technically can replace larger extinguishers if the area calculation is satisfied. However, what people are not being informed about with these 3 litre extinguishers is that the discharge length (the distance the spray will reach) is much reduced, meaning you will need to stand closer to the fire. Harping back to the manufacturing standard for extinguishers before EN3, which was BS5423, in that standard there used to be a requirement for extinguishers to achieve a certain discharge length; it was 6 metres for water and 4 metres for foam. This requirement was removed in EN3 and that's why we have seen water 'spray' extinguishers being introduced since they no longer need to hit 6 metres. This also means that you can have a 3 litre foam extinguisher that has a very fine spray nozzle, which allows for a good fire rating, BUT at the cost of discharge length (since the 4 metre distance requirement was removed) meaning you need to be closer to the fire. It also requires that the same high performance foam concentrate is required at the 5 yearly extended service and engineers just won't carry that.
We won't supply 3 litre extinguishers to clients and recommend 6 litres as a minimum. Sure 3 litre extinguishers 'tick the box', but when it comes down to it, we want to provide kit that the user can actually make use of if required.