According to various dictionaries, when used as an adjective, plump means having a full and rounded shape, whereas round means circular or cylindrical. Of course those of us whose shape is not exactly lean are used to being referred to as being plump [in the sense of having a full and rounded shape or being chubby and somewhat overweight.] Should you take offence at this description you might quickly be assured that ‘plump’ in this case is meant to describe an ‘appealing roundness’. Let us look at some examples of creatures with this appealing plumpness.
I don’t think I have ever seen a dassie that looks thin!
Olive thrushes always look ‘cheerfully chubby’.
Unless they have very recently been shorn, sheep also have an appealing roundness about their appearance.
As far as round things go, look at the spherical shape of this dung ball – courtesy of the work of a dung beetle.
Dandelion seeds are appealingly round.
The shape of these rings on a gun carriage may generate a discussion on the difference between round, circular and spherical – we can leave that for another day!