I have always been more comfortable with using a calving ease Angus bull on heifers of any breed, especially Angus type heifers. I am sure that there are some consistently low birthweight calving ease Hereford bulls around, but we have had really good success with Angus bulls for heifers. This spring I did calve out some purebred Hereford heifers to our moderate BW Hereford bull, with no issues at all, but wouldn't have wanted to ideally breed any heifers of other breeds other than Hereford to him.
I do think that hybrid vigor does play a role, but our heifers have all had calves on their own regardless of breed when bred to CE Angus bulls, except for one breech birth a couple years ago.
In the case of the post where huge calves were coming from Angus bull x Hereford heifers, its something that could happen for probably a number of reasons or combinations. If the heifers were from large BW parent stock, perhaps the bull wasn't a CE bull. Condition could also play a role. Prior to or without DNA parentage testing i'm quite sure there have been a number of mistaken genetic identities. My first Angus bull was not a CE bull, didn't realize back then that not all Angus were CE, bred him to some Charolais heifers and had a fiasco almost like what the fellow with the Hereford heifers had. After that and some research I got onto CE Angus bulls for heifers and haven't looked back.