Below are dims from my Mk1s.Newer ones the same huge size. You can't estimate the volume of the flexible bag the way you can a rigid box. The true volume will be more (like a flat envelope has technically zero volume but as we know can fill out) More here. (may have gone through all this earlier in the thread, years ago) How secure when mounted depends what you mount them to. Jut throw them over like horse and they'll flap around. Secure front and rear of the bag to a rack or pillion pegs - is better. The Mk2s have ring tabs to do this. Or do you mean theft security? Well, a committed thief with a sharp knife could cut the straps off the bike. The actual aramid liner n the body is tough and stab proof. Even a red-hot nail had trouble pushing through, I found. Was recently using some Kriega Duos. Handy size but the over-complex flap closure drove me nuts! Mag simplicity and function is the best.