That’s actually good point, especially for recordable discs. As for the bulkiness, they wouldn’t take up any more space than a regular disc in a jewel case. I remember using a DVD-RAM enclosed in a cartridge back around 2002, so it’s not like that the disc cannot operate inside an enclosure. That LaCie DVD writer at the time could accept both bare and cartridge based discs. As both sides of the disc would be protected, they could even be made double-sided.
As for the BD-R/RE XL, the only BD-RE XL disc I could find was a Panasonic BD-RE Triple-layer 100GB on eBay UK, priced at Â£79. For a mere Â£1 extra, I can get a 128GB Lexar USB stick on Amazon, which is not affected by scratches, fingerprints, warping, being left in the sun, etc., has 28GB of extra capacity and doesn’t not require an optical drive. As for size difference, try placing the BD-RE XL in a jeans pocket.