I`m sure you will understand everything
. It is a simple 5-minute setup.
I have separated each step by new transforms instead of forcing everything into one or two.
Just pay attention to the SplinePosition parameter: It will take in count XSplineposition as it is from 0-1 but I calculated both sizes at the same time so input values should be less than 0.5
Please check BaseSpline Modifier also for some banking variations.
You could also use markers to separate different Scale Sizes, but I`m sure you could do that on your own. (just add markers and sed the IDs from 1-3 and for the Selector choose X Spline Marker ID)
With some math, you can automatize everything Scale+Paddings. Sorry I didn`t have time to play more.