Hercules beetle grub – Identification
Q: Is this size of grub common?
A: In the Amazon those grubs would be snack size for the local people. I have eaten grubs there and found them tasty.
Indigenous families grow the chonta palm, Bactris gasipaes, in spots near their homes. When a palm falls they chop holes in the trunk for beetles to enter and lay their eggs. The eggs hatch and turn into grubs which consume the center of the tree. When a native wants a snack they pull the grubs out with their fingers and roast them with a little salt.
That said, your grubs are probably Hercules beetle, Dynastes hercules, grubs. They over-winter in rotten logs in the landscape.