[Google Pixel Questions & Answers] Pluisje89:
Somehow I couldn't copy my music to my phone. Decided to check it out...

So the system image is using 109GB instead of 5-10GB as it should

Running the latest factory images NOF26V, freshly installed

finished. total time: 1.235s
MacBook-Pro-2:android iichel$ ./fastboot flash bootloader bootloader-sailfish-8996-012001-1611091517.img
target reported max download size of 536870912 bytes
sending 'bootloader_a' (32980 KB)...
OKAY [ 0.843s]
writing 'bootloader_a'...
(bootloader) Valid bootloader version.
(bootloader) Flashing active slot "_a"
(bootloader) Flashing active slot "_a"
OKAY [ 15.964s]
finished. total time: 16.807s
MacBook-Pro-2:android iichel$ ./fastboot reboot-bootloader
rebooting into bootloader...
OKAY [ 0.046s]
finished. total time: 0.046s
MacBook-Pro-2:android iichel$ ./fastboot flash radio radio-sailfish-8996-012511-1611190200.img
target reported max download size of 536870912 bytes
sending 'radio_a' (57192 KB)...
OKAY [ 1.403s]
writing 'radio_a'...
OKAY [ 0.910s]
finished. total time: 2.314s
MacBook-Pro-2:android iichel$ ./fastboot reboot-bootloader
rebooting into bootloader...
OKAY [ 0.046s]
finished. total time: 0.046s
MacBook-Pro-2:android iichel$ ./fastboot flash boot boot.img
target reported max download size of 536870912 bytes
sending 'boot_a' (24801 KB)...
OKAY [ 0.652s]
writing 'boot_a'...
OKAY [ 0.611s]
finished. total time: 1.263s
MacBook-Pro-2:android iichel$ ./fastboot flash cache cache.img
error: cannot load 'cache.img'
MacBook-Pro-2:android iichel$ ./fastboot flash system system.img
target reported max download size of 536870912 bytes
erasing 'system_a'...
OKAY [ 0.995s]
sending sparse 'system_a' 1/4 (515466 KB)...
OKAY [ 15.869s]
writing 'system_a' 1/4...
OKAY [ 7.944s]
sending sparse 'system_a' 2/4 (495893 KB)...
OKAY [ 15.867s]
writing 'system_a' 2/4...
OKAY [ 10.045s]
sending sparse 'system_a' 3/4 (507843 KB)...
OKAY [ 15.358s]
writing 'system_a' 3/4...
OKAY [ 7.454s]
sending sparse 'system_a' 4/4 (32756 KB)...
OKAY [ 0.904s]
writing 'system_a' 4/4...
OKAY [ 0.651s]
finished. total time: 75.089s
MacBook-Pro-2:android iichel$ ./fastboot flash system_b system_other.img
target reported max download size of 536870912 bytes
erasing 'system_b'...
OKAY [ 1.043s]
sending sparse 'system_b' 1/4 (517080 KB)...
OKAY [ 14.369s]
writing 'system_b' 1/4...
OKAY [ 4.792s]
sending sparse 'system_b' 2/4 (514384 KB)...
OKAY [ 14.517s]
writing 'system_b' 2/4...
OKAY [ 4.998s]
sending sparse 'system_b' 3/4 (524287 KB)...
OKAY [ 14.612s]
writing 'system_b' 3/4...
OKAY [ 4.540s]
sending sparse 'system_b' 4/4 (85786 KB)...
OKAY [ 2.549s]
writing 'system_b' 4/4...
OKAY [ 1.201s]
finished. total time: 62.622s
MacBook-Pro-2:android iichel$ ./fastboot flash vendor vendor.img
target reported max download size of 536870912 bytes
erasing 'vendor_a'...
OKAY [ 0.503s]
sending 'vendor_a' (235186 KB)...
OKAY [ 5.849s]
writing 'vendor_a'...
OKAY [ 6.707s]
finished. total time: 13.060s
MacBook-Pro-2:android iichel$ ./fastboot flash modem modem.img
target reported max download size of 536870912 bytes
sending 'modem_a' (57184 KB)...
OKAY [ 1.431s]
writing 'modem_a'...
OKAY [ 0.849s]
finished. total time: 2.280s
MacBook-Pro-2:android iichel$ ./fastboot flash userdata userdata.img
target reported max download size of 536870912 bytes
erasing 'userdata'...
OKAY [ 29.416s]
sending 'userdata' (136402 KB)...
OKAY [ 3.444s]
writing 'userdata'...
OKAY [ 1.159s]
finished. total time: 34.019s
MacBook-Pro-2:android iichel$ ./fastboot reboot
rebooting...

Found a similar thread, but a factory reset didn't fix my issue either:

https://xdaforums.com/pixel/help/7-1-1-taking-109gb-storage-t3522967