Whe I make curry (either from scratch or from a jar) I always try to bulk it up to make enough to be able to freeze a portion or two. I always boil up a pan of water with potatoes, carrot, turnip, parsnip - any veggies really, cook them for 10 minutes or so, drain (but not completely!), blitz up and add to the sauce. Bulks the meal out, makes it filling and tastes fab.
You can still add your onion, peppers, leek, sweetcorn etc in 'pieces', so you can bite them.
Works well with bolognese too to use carrot, lentils etc. A great way to disguise veg if you've got fussy kids too!