Question: Why have the list of references increased in size for a Medline record?

Answer: The NLM has a new policy for references included within Medline documents. 

Here are the changes
  Not all MEDLINE documents have references in the style of the new example; there are still plenty in the old style. 
Not all MEDLINE documents have references at all.  About 18% of documents in MEDLINE have references.

The new style makes the space needed to display the references much larger than before.  We have examples of documents where the references take up 10 pages or more.  See example an(31657050)
Documents delivered by the alerts are much longer than they used to be and difficult to review.

On live Dialog, customers can avoid using the Citation/Abstract format and instead choose Custom to exclude the References. In the alert output, it is currently not possible to select/unselect References with the Alert custom format. We have requested an enhancement of the Alert Custom format.

The NLM does not provide the references in separate elements so we cannot present them exactly as we do for SciSearch, but we are pursuing some display improvements, including DOI live links.

Document Status affected.
The NLM has told us that "Publishers may now submit their reference lists to PubMed. Previously this feature was only available for PubMed Central articles."
So publishers can submit references when they provide the earliest form of a record to the NLM – ‘Publisher’. 
It looks like the NLM is retaining the list through the various statuses of the record from Publisher, In-Process to full Medline. 

Customers will have seen lots of these mostly on Publisher records (no indexing), because the NLM is accepting them in this form for the first time.  As the year progresses and the documents move into later status types, we will see these reference lists on all types of Medline records.  This means that using the document status to exclude or include these references will not be a good workaround.