Can cooking in a larger than intended vessel be disadvantageous in some way? I have a big pressure cooker (3.5 litre) in which I cook 3/4 cup Lentils (on gas stove) intended for 2 people. Can it be disadvantageous in some way? 

Are there some serious reasons that I should consider buying 1.5 litre pressure cooker to cook that amount of food?
Every pressure cooker has a minimum amount of liquid which it must be filled with, in order for that liquid to evaporate and provide the pressure increase.

Check the manual for your pressure cooker to ensure that the small quantity of lentils you want to cook is at least the minimum.

If it is not, you can either prepare more lentils (hopefully you can use the leftovers), or perhaps get a smaller pressure cooker.

Of course, it doesn't take very long to cook lentils conventionally, so when you want only a small amount, you might consider cooking it in one of the traditional stove top or oven methods.