switching frequency is usually fixed. (depends)
In other words, to get a constant I load, you have to switch ON and OFF, and current across your inductor is going high (when ON) and going low (when OFF) like a triangle wave, then you filtered it with a capacitor to get the so called 'constant' DC voltage with ripple.
when I load is higher, in switched mode power supply, your ripple current across your inductor also higher, thus your ripple voltage also higher as well.