Pizarro's arm isn't comparable to Thriller Bark, it was as big as a Mountain as Oda explained this chapter.
Even Kaidou's airslashes cut Mountains.
Pica was even more insane, he had literally towns on his palm.
Just look at the buildings on Pica,
Pica's
arm > Pizarro's arm.
The scale itself is vastly different.
That thriller bark scaling is just Oda failing at perspectives & scales. Because he wanted to zoom in on the scale - he later clarified it was a mountain.