<BIG> </BIG>

The BIG tag will make the text slighter larger than normal. The FONT tag can do the same, with the SIZE="+1" attribute. Nesting BIG tags will produce text in a larger font than with just one BIG tag. There isn't a problem with nesting the BIG and SMALL tags, but we don't see why you would want to do that.

<big>This text is BIG, </big> but this text isn't.
This text is BIG, but this text isn't.

Nesting the BIG tag looks like this:

<BIG>BIGGER <BIG>and <BIG>BIGGER <BIG>and <BIG>BIGGER </BIG></BIG></BIG></BIG></BIG>
BIGGER and BIGGER and BIGGER