We live in a 3 bed with an open plan living, dining and kitchen, 2 bathrooms and a rumpus/second living area. This is the perfect size for DH, DD and myself, And we are building onto our deck this summer and making a specific childs play area/playground in the average size yard so I foresee no problems spacewise for us even once we have added to and completed our family. Seriously if we did move to a bigger house it would solely be for selfish 'I want' reasons and not real 'need' (not saying others with big houses are selfish or dont require them just that for us we have more than enough space to live 'comfortably' with our planned 4 - 5 member family).
Op, I say do what you have to do and if finances mean you have to stay in the 1.5bed unit for the twins first 12months then so be it. Im sure you can make it work but I assume massive organisation and not much clutter will be key to it being successful. I wish you the best of luck!