iCloud "message size violation" when sending zip containing more than 127 files Revised question
As per Tetsujin's suggestion in the comments I did some further testing.
There seems to be a problem with iCloud and sending a zip file of a folder with a certain amount of files. There appears to be a limit of 127 files in 1 folder, regardless of file type (tried .png, .svg and .txt).
When sending the mail using the Mail app I get a message size violation error and the mail can not be sent. Using the iCloud web interface the mail does appear to be sent (no error, appears in 'sent' list) but it never arrives.
Does anyone know why this limit exists? (as Tetsujin mentioned in the comments it might be some 7-bit shenanigans)

Old question
If I try to send an email with a 1.3 MB zip file as attachment I get a message size violation error:

File sizes of the zip file and original folder (1.3 MB and 10.7 MB respectively):

At first I thought it was somehow related the original folder being more than 10 MB, but according to Apple's iCloud support page the limit is 20 MB:

Limits on sending messages
There are several safeguards in place to make sure that only iCloud members can send messages using iCloud. One of these safeguards includes reasonable limits on:

...
The size of incoming and outgoing messages (20 MB, up to 5 GB with Mail Drop turned on)


Why am I unable to send this zip file? (also: shouldn't Mail Drop kick in if it thinks the attachment is too large? It is turned on.)
I just ran into the same problem : zipped two 11.5 MB Xojo compiled apps. Sending from me.com or sending to me.com gave the 'message size violation' error. 

To bypass this possible 127-file limit, I created an empty disk image with Disk Utility. Mounted it and copied the two apps onto it. Unmounted and zipped it and sent it via me.com : that worked.