I have heard that Weatherbeetas are quite generous with their sizing, but haven't come across any brands specifically for the chunkier horse. My friend has a very heavyweight cob (16.1hh) and I think most of his rugs are Derby House or Weatherbeeta.
The Premier Equine rugs are deep & roomy around the shoulders. I've got an ID who's quite wide & some rugs look like greyhound jackets on her but the Premier ones come down to her knees. Sign up for their email newsletter as they often do 24hr sales
Go down a size in Premier Equine. My chunky boy's 5'9" Weathabeatas he came with are a touch small. I bought 6' Premier Equine rugs and they swamped him. Seriously, looked like a kid on the first day of term in a uniform his Mum says he'll grow into!
You'll hear different recommendations from everyone you ask As an example, Amigo rugs are great, but I've always found them quite tight round the chest - no good for chunks.
Shires are good (and deep) - the stormbreaker rugs fitted my HW cob really well. Rhino was also good (though not as deep), and Fal wasn't too bad.
New boy has a Weatherbeeta Freestyle LW at the minute, and he can't fill the chest up at all - I imagine it'd be brilliant on a heavy.