This is a great script for checking what page types are causing bytes sent
EXTRACT_EXTENSION(cs-uri-stem) AS PageType,
SUM(sc-bytes) as TotalBytesSent,
TO_INT(MUL(PROPSUM(sc-bytes), 100.0)) AS PercentBytes
Let say your CSS file had a lot of TotalBytesSent you might want to check these or if your pictures was high in % you might want to reduce the size/resolution etc.