Magnified Effect support gem not working correctly

I have Witch lvl 12. When I place Magnified Effect to support Skeletal Arsonist, the explosion radius for Explosive Demise moves from 2m to 2.3m. Magnified Effect gem state that it must increase AOE with 40%, from 200sm to 230sm is just 15%. New explosion radius must be 2.8m.

Just tested and Skeletal Sniper Impact radius of Gas Cloud is changing from 1.4 to 1.6 which is just 15%, not 40%.

Tested also with Contagion and here Spread Radius jump from 1.7m to 2m, this is an increase of 17.6% again not 40%, which is strange compared to my first two tests.

Tested with Bone Cage and here Cage Radius move with 16.6% from 1.8m to 2.1m, explosion radius with 10% from 1m to 1.1m, again nothing close to 40%

With just around 10% to 15% increase in the AOE radius, this support gem is trash right now.

You're doing the math wrong. Magnified Effect gives you 40% more area of effect, which is the area of the circle that these effects have. You are looking at radius increases, which is not the same.