I didn't have much idea about Data- proprietary in Pega as specific. Though, I found another article having below information. You can refer it, there is a pdn document link as well. I am not sure, How much it would help you.
You can try with compression parameter set in prconfig.xmlDeflateStreams to "true.
The below link has information on LOB sizing optimization.
Here is a point on compression from the same document.
"Compression, which can reduce BLOB size by a third or more, is enabled by a property setting in the PRPC pegarules.xml file (versions 4.x and earlier) or the prconfig.xml file (versions 5.x and later). Compression is turned on by default. Refer to PRKB-9850, How to Compress the BLOB Values in the PegaRULES Database, for further information."