Perhaps one way...

On the sheet with more rows (let's call it Sheet2), enter in a new column, say J2 a formula to determine if the item number matches that of the sheet with less (let's call it Sheet1), the formula would be like:

=Isnumber(Match(A2,'Sheet1'!A:A,0)) copied down

TRUE result means item found.

You can then apply Autofilter from the Data menu and filter For FALSE to see which didn't match and do further work from there.....