When I 1st got both my yorkies I was told they would not grow over 4lbs they are now both close to 5 and I dont notice a difference in size from when they were 4.. 5 & 6 lbs. I too would be upset about being lied to but in time you wont be upset about the size of your yorkie, I am sure you will love your yorkie unconditionally no matter what size she ends up
I believe Maltese and yorkies have different bone structure so a 6lbs maltese may look larger than a 6lb yorkie. You never know, they might end up looking the same size or the yorkie even looking smaller.