Pickle in the UK means anything preserved in vinegar, which may or may not be spiced, from whole eggs and onions to cucumbers, gherkins and other vegetables (which are either cooked, blanched or raw and then put into vinegar), to various fruits and vegetables chopped up and cooked in the vinegar with sugar and spices (what I think you mean by relish and what we should probably call a chutney). Preserve is a jam (jelly) or marmalade.
When you think about Frank's jars and their possible contents, it starts to make sense. Chopped up squishy stuff in the narrow chutney, whole or bulky things in the fatter pickles (those with tasty vinegar in one with a spout). Here the ground tops keep the vinegar smell in. Preserve in a covered jar to keep the dust out or on the compote (nappie). These were all for serving - not for preserving. Of course it was a marketing ploy. Why sell someone one all-purpose pickle jar for all pickles when they might buy three different ones.
When you think about it how many sizes or plate do you really need? Most of us have more than one!