Here's something fascinating that Anand Katakam found when he was researching State of the Union speech lengths — Democrats give much longer speeches:
The famously verbose Bill Clinton pulls up the average, obviously, but Obama is a long talker as well. And presidents John F. Kennedy and Lyndon B. Johnson both gave speeches that were longer, on average, than the average Republican address.
Obviously the sample size here is not enormous, so it could just be a coincidence. But I think this reflects something that a substantial body of research has found — Democrats and Republicans are genuinely different, with Democrats caring more about policy and Republicans caring more about philosophy. The gap in interests is re-enforced by a gap in public opinion, where conservatives outnumber liberals but virtually no specific policy program is unpopular.
Both dynamics help encourage Republican presidents to keep things brief. Rather than a detailed agenda for small government and free enterprise, a Republican is well-served by offering a relatively concise high-level view of the virtues of small government and free enterprise.
A Democrat, by contrast, serves both his own inclinations and the tastes of the public by diving deep into details of specific programs where he thinks government activism can make people's lives better.