Why is the micro USB so fragile compared to mini USB cable ends?

I bought a bunch of micro-USB cables recently from Frys, and
almost half of them have had the two little nubs on the top
of the male micro-USB connector disappear.

I don't know if they fall off, or if they get pressed down,
or if they bend, or what, but the cables just fall out of
the various devices I use them in (earbuds, smart phones,
speakerphones, etc.).

Is it that micro_USB is inherently more fragile than mini_USB?
Or is it just bad cables from Frys?