The blur operations were clamping the filter size to 1, which prevents
no-op blur nodes. Further any value < 1 would also be ignored and in
many combinations the filter scale setting ("Size") would only work in
integer steps.
Now most blur settings will work with smooth Size value scaling as well,
meaning you can choose a reasonably large filter size (e.g. 10) and then
use the Size factor to scale the actual blur radius smoothly.
Note that non-integer filter sizes also depend on the filter type
selected in the Blur node, e.g. "Flat" filtering will still ignore
smooth filter sizes. Gaussian filters work best for this purpose.
