A marine dish for DIRECTV HD is big.
It's about a foot wider than a dish for DISH Network or DIRECTV SD. We're talking about something that's really too big for anything but a larger craft. Why is that?
DIRECTV's HD broadcasts are in a frequency range used by no one else in North America. This "Ka Band" is licensed exclusively to DIRECTV, and it was a big deal when they first did it over a decade ago. This allows DIRECTV almost unlimited expansion into HD and 4K, giving the ability to broadcast thousands of HD channels with no concern for interference.
In contrast, DISH uses the same Ku Band that everyone else uses and as a result they sometimes deal with issues of channel capacity, which mean fewer channels in fewer markets. But there is an upside to being in that crowded range where all other satellite broadcasts live: it's a better place to be in a lot of ways.
The Ku band is less susceptible to interference from clouds and rain so it's possible to get a stronger signal with a smaller dish. In order to get a strong signal with DIRECTV's Ka Band even on a rainy day, a bigger dish is needed. So, DIRECTV dishes tend to be bigger, simple as that.
With DIRECTV phasing out standard definition service in 2019, manufacturers like Intellian, KVH and Seatel are working hard on ultra-sensitive electronics and low-noise amplifiers that may allow people to use those smaller domes and still get HD service. It's a race with the clock to be sure,