No one can say as we don't know what the update relates to.
If for instance you have JAVA still installed this is a largish update. Then there is flash player that is frequently updated as it is a huge target for malware and that has two updates IE & plug-in based browsers, both of these weigh in at about 15MB each.
So we don't know from the information given of A) what programs on your system avast S.U. is monitoring or B) which required updating.