Actually on my mare it depends on whether it is stable or turnout. I find weatherbeeta stable rugs enormous, but her 6'3 WB turnout fits the same as her 6'3 amigo
The only thing is the amigos are not adjustable at the chest, so if your horse has a big build, you may find them smaller, relative to the weatherbeetas if you see what I mean!
I don't think they are particularly narrow - but it depends entirely on the horse! My mare is a cob x WB but doesn't have particularly massive shoulders, and they fit her perfectly.
However, a friend has a very narrow connie x tb and they fit him well too - I think they are only a problem if you have a very stocky horse.
I have two ponies who both wear 5' rugs...the "leg at each corner" welshie is more comfortable in the weatherbeeta, the mini thoroughbred type fits the Amigo better....
The Amigo rugs are narrow in front (they fit my 20 month old well)
The Weatherbeetas with the original side gussets (not the Freestyle ones) fit my chunky Welsh x TB who is wide in front.
I bought a Weatherbeeta stable rug for my girl (very narrow in the chest)l, it was HUGE!.
Normally she is 6'3'' but i had to change it to a 6'. It was still too big but because of the hassle the first time with the shop assistant and the fact it was very cold I decided to keep it. It ended up rubbing her shoulders and in the end i bought an Amigo which fits her perfectly, and she no longer has bald patches on her chest!
So I would say that Weatherbeeta stable rugs are very very generous and great for a chunky monkey.
I only use outdoor rugs on my boys as their stable doors are open 24/7 to avoid weaving, box walking or handstands!
They have Weatherbeetas the TB has 6ft 3 Weatherbeeta Outlanda style (old models now) and WB X takes 6ft 9 rugs in both 6oz and no fill; however the darts are on the sides. They do up nicely at the chest on the middle holes and have plenty of room in for them to do their cartwheels when in the field!
I did purchase the newer style Weatherbeeta Freestyle in same sizes 6ft 3 and 6ft 9 no fill turn out rugs but with the very short darts at the front of the chest. They are really quite short in the chest and only do up on the 1st or 2nd holes. I will probably not buy this style again as they are far too fitted and less roomy around the shoulder perhaps I should have brought the next sizes up!
I dont use Amigo rugs so I cannot comment; however my boys have also got the Masta Tri-vent Medium to light turnouts in 6ft 3 and 6ft 9 and are done up on the tightest holes at the chest the rug overall is huge on both my boys (especially around the shoulders) and I think I really could of gone down a size but then the length from the withers to the tail may not be long enough.
When the rugs are soaked (rain) the Masta is a lot heavier than the 6oz Weatherbeetas.
If it helps at all Prince (connie) takes 6'3 Amigo's. 6'3 weatherbeetas are fine length wise on him but are too big round the shoulder so they sit low and rub. Badger (Fjord) and Bodey (Cob) wear weatherbeeta's and they fit nicely, the amigo's are too tight round the shoulder for them.
Weatherbeeta def wider fitting, and therefore often needs a size smaller.
I've found the amingo's aren't too bad width wise (though too tight for very broad chests) but if you're looking for a narrow rug get the Horsewear Rhino or Rambo (ones with the V-closure) as they fit really tightly and small.
Amigo rugs in my experience come up a size smaller than other rugs including other Horseware ones, so all my Amigos are a size larger than my other rugs.
Don't like anything other than Horseware, so no recent experience of Weatherbeeta.