In the episode Mermaid Man and Barnacle Boy IV, Patrick sets Spongebob's belt from Mini to Wumbo. After Spongebob shrinks Patrick, his belt is set at Mini again.
When SpongeBob is running to Patrick's rock, the belt is missing; in the next shot, when he is talking to Patrick, Mermaid Man's belt re appears.
When SpongeBob's friends are shrunk, they're all roughly about twice the size of Plankton. But when Plankton returns from a vacation to Bikini Bottom, he's a giant.