Matryoshkas are often weighed to follow a particular theme, for instance peasant girls in traditional dress, but the theme can be anything, from fairy tale characters to Soviet leaders.
The onion metaphor is of corresponding character. If you peel the outer layer off an onion, a similar onion exists within the outer layer. This construction is employed by designers in applications such as the layering of clothes or the design of tables, where a smaller table sits within a celebrated table and a yet smaller sole within that. See also onion routing.
