I'm going to re-post your Mars nutrition so it's out of 100g (makes it easier to compare) to my go-to, Soreen.
1,888kJ/449kcal energy, 4g protein, 69g carb (60g sugar), 17g fat
Soreen (per 100g)
1222kJ/288kcal energy, 7.5g protein, 60g carb (22g sugar), 1.6g fat
Gram for gram, I would say Soreen is better since it gives a good protein/carb ratio (plus less sugar) for longer-lasting energy. Size-wise, of course, it's five times larger (though one could always cut it before going on a trip), but it can survive hell and back again and still be delicious (and squidgy).
I'm not sure you'd want to go much smaller than 50g, since anything much less than that and you won't be really getting enough food to make it worthwhile. I'd buy the Soreen slices and just chuck a few in your pack.
EDIT: I should mention there are higher-calorie/protein options out there, but they're from health food stores and four times the price. I didn't really think to say anything about it because (personally) it's not worth the extra money.
EDIT 2: to not double-post, Pete's right, a Snickers is better than a Mars (more protein/energy, fewer sugar carbs), and beats out Soreen by a fair amount.