You are correct that there no longer is a way to configure the storage limit of logs. Logs are simply stored for 30 days regardless of the storage space occupied as far as I know.
Regarding the gzip encoding for Stackdriver Logging exports to Cloud Storage, this sounds like a great idea and I highly recommend that you file a
feature request on the Cloud Platform public issue tracker and post a link to it here so that others may follow up and support the request.
In the meantime, if this compression is really crucial, I suppose one could create a cron job that retrieves, compresses and re-uploads logs to Cloud Storage but I suspect this would not be very cost effective as the handler to perform this daily task would cost instance hours (albeit fractionally).