"Fuck, if we've already begun a grammatical witch hunt, shouldn't it be "bland yet copious?"
The way I usually think about it is to decide whether the words are opposites in some way. More specifically, if something is flavorless, would you expect it to automatically be a certain size? Not really... blandness unfortunately occurs in all shapes and sizes of food. A bland thing could be minute or it could be copious. Copious isn't the opposite of bland.
"Secondly, how can lettuce be anything other than bland? It's a vegetable made up largely of water!"
The cliche "you are what you eat" has some truth to it. Animals who eat grass vs. corn produce beef with different flavors. That's part of what these new burger joints are attempting to use. Vegetables eat nutrients and chemicals from soil. A popular old science project for kids used to be putting a white flower in a glass of water with food coloring and watching it change color. If vegetables grow in soil that's full of awesome, they are more flavorful. If they grow somewhere crappy, they have a different texture and taste. I grew up believing that all vegetables were bland, flavorless piles of disgusting mush. My mom bought whatever she could afford at the local chain grocery store and it usually came from a can.