<BIG>

<BIG> makes the text bigger (makes sense, doesn't it?).

this code	produces this
This is a really <BIG>big</BIG> idea!	This is a really big idea!

<BIG> doesn't really produce text that is all that BIG. For a more pronounced BIG effect it's much better to use styles. For example, we could use this styles code (in a STYLE tag or in a style sheet file):

.bigletters
{
font-size:400%;
}

and then apply it with a <SPAN ...> tag like this:

this code	produces this
This is a really <SPAN CLASS="bigletters">big</SPAN> idea!	This is a really big idea!