By global average, under 15% of farm revenue is derived from government subsidies. USA is below that, at about 10%. Not sure if I'd call that massive, but that's semantics so it's a little hard to argue against. Does potato agriculture get massive subsidies?
> If all subsidies were removed - in order to avoid the influence of moralising politicians - people would eat a lot more potatoes, and a lot less beef.
The assumptions being that 1. potato farming get relatively much less subsidies as beef (and other meat) farming; 2. cost is such a factor in consumption that price change would cause "a lot" of difference. I don't think either are very safe, and as a general statement it doesn't follow that just reducing agricultural subsidies increases ratio of beef to potato (or meat to vegetable): EU subsidies are much higher than US, but USA eats far more beef per capita.