It's a lot cheaper to buy extra veg to make chicken last for two (or three) meals than it is to get a takeaway for the same period whilst having lunch of a packet of crisps, a cheese sandwich with a Red Bull and pop tarts or a Snickers for breakfast.
The comparison is in the variety, rather than the volume - somebody having small amounts of different plants is consuming a wider range of nutrients and fibre compared to somebody whose weekly menu consists of jarred sauces + meat, toast with spread and every other meal being served with potatoes, peas and sweetcorn.
It's a version of the reasoning following research showing that people eating a traditional Japanese/Mediterranean diet experience on average greater longevity and better health than those who have a more restrictive range of foods and fewer vegetables. A meal in those styles is characterised by multiple vegetables in addition to grains, meat and fish (and fermented products). Japan recommends consuming 350g of veg and 200g of fruit per day to meet nutritional needs, for example - and although it's true that target isn't met for many, even their lower actual consumption is considerably higher; chowing through 350g of spinach isn't realistic or good for anybody, but 250g of multiple types in a day is more likely.
You're more likely to reach a higher intake with encouragement to eat multiple things, as if nothing else, they are attractive and appetite is affected by visual appeal as well as taste and smell. And it's easier to chop up/grate a courgette or some broccoli to add to a tomato based sauce between 4 than it is to get 4 people to eat some boiled green stuff on the side of a plate, fighting for space between the roasties, yorkshire puddings and mash.
I don't have any issue with the angle they're taking - multiple small portions is more likely to increase intake (and volume could well increase over time as people become accustomed to automatically adding extra plants to each meal and enjoy them, thereby reducing consumption of less nutritious meals or components).