FosseWay's comments refer to the Swedish coinage. Now I wonder why Sweden did that?
Value 10 Kronor
Metal Nordic Gold
Weight 6.6 g
Diameter 20.5 mm
Thickness 2.87 mm
Shape Round
Year 2016
Value 5 Kronor
Metal Nordic Gold
Weight 6.1 g
Diameter 23.75 mm
Thickness 1.97 mm
Shape Round
This is pure speculation, but I wonder whether it's the result of a change in plan halfway through. The new-spec 5 kr would have worked fine if it was in white metal - it wouldn't have been in the same "family" as the 10 kronor then, and the relationship wouldn't have been any more odd than the fact that the UK 10p is wider than the £1 or the 5 eurocents wider than the 10c. I wonder, therefore, whether the original plan had been for a white-metal 5 kr, but this was rejected because of allergy issues. For reasons I'm not clear on, there has been an undercurrent of complaint for some time in Sweden about the unsuitability of nickel as a coinage metal because of allergies. Never mind that the "nickel" in the previous coinage was actually mostly copper, or that everywhere else in Europe manages fine with cupro-nickel, or that a number of countries have or have had pure nickel coins without apparent problems. Nickel is a heavy metal that is toxic, never mind allergenic, if ingested and introduced into the circulatory system and organs. So is copper, but there isn't a similar outcry in that case. And ingesting coins is hardly a widespread issue, I'd have thought.