I think that sounds like something called a blighted ovum (empty gestational sac), where the fetus does not continue to develop past the 6th week.

However, it does seem really early on in the pregnancy and up to week 6-7 the u/s might only show the sac, and no developing fetuses.

Is it possible to have another u/s in two weeks to check?

As to the size issue, i agree with satya - twins/triplets can often develop at different stages, and sometimes only one actually develops.