If the replacement you've been offered is close to being like-for-like then I would suggest you should accept it (or request your old one back).

If the replacement you've been offered would not be considered by a reasonable person to be like-for-like then I would insist that they make good on their original offer. I've not read the detailed discussions about the dimensions but if the volume of the new one is considerably less than the old one then I fail to see how it may be considered like-for-like.