In reply to mypyrex:
Did you cut one of the giant lemons open and see how thick its skin was?
Different varieties have different characteristics. Different thickness skin, aroma, sourness, bitterness and flavour.
You'll probably find that the buyers for the UK supermarkets bring in one or, if you are lucky, two varieties of lemon that have been selected for their customers' perceived tastes - Thin skin (lots of juice for the weight of lemon), sour but not too bitter etc, etc.
Thin skin, small lemons are great for a slice in your G&T.
The ones with thick skin work really well for preserved lemons etc, but how many supermarket customers buy lemons for that?
tldr;
Supermarkets offer a small range of produce, whilst claiming otherwise.
Post edited at 20:08