Some corrections:
This is back down to 1.5.
Widow Mines have a 1.75 radius.
Disruptors have a 1.35 radius. The Disruptor’s strike radius has never been larger than 1.5.
1.25 max. The Siege Tanks’ splash radii are split into 0.4687, 0.7812, 1.25 for 100%/50%/25% damage respectively.
It takes several hits for Siege Tanks to deal as much damage as Widow Mines (especially against non-armored targets), and that is one of the reasons why Widow Mines tend to be the better option against Zergling/Baneling and a few other compositions.
1.5. Not 1.75.