The large bug is the female and the smaller one (pictured twice) is the male. They are only about the size of a plant bug - maybe a bit bigger - but quite a bit smaller than other stink bugs (of the family Pentatomidae) which the bagrada bug is a member of.
I have a pretty small garden in the middle of suburban Fresno and it must have been a very lucky pair of bugs that found my lettuce plants. That, or maybe mizuna lettuce is just irresistible to the critters. You have to admit, they're pretty colorful.