The MI_VISIBLE_PARTITION structure exists only as a component of the MI_PARTITION. It is 0x0B80 or 0x1100 bytes in 32-bit and 64-bit Windows, respectively.

Offset (x86) Offset (x64) Definition
0x00 0x00
ULONG_PTR LowestPhysicalPage;
0x04 0x08
ULONG_PTR HighestPhysicalPage;
0x08 0x10
ULONG_PTR NumberOfPhysicalPages;
0x0C 0x18
ULONG volatile NumberOfPagingFiles;
0x10 0x20
MMPAGING_FILE *PagingFile [0x10];
0x80 0xC0
ULONG_PTR volatile AvailablePages;
0xC0 0x0100
ULONG_PTR volatile ResidentAvailablePages;
0xC4 0x0108
ULONG_PTR volatile TotalCommittedPages;
0x0100 0x0140
MMPFN_LIST ModifiedPageListHead;
0x0140 0x0180
MMPFN_LIST ModifiedNoWritePageListHead;
0x0154 0x01A8
ULONG_PTR TotalCommitLimit;
0x0158 0x01B0
ULONG_PTR TotalPagesForPagingFile;
0x015C 0x01B8
ULONG_PTR VadPhysicalPages;
0x0160 0x01C0
ULONG_PTR ProcessLockedFilePages;
0x0164 0x01C8
ULONG ChargeCommitmentFailures [4];
ULONG_PTR volatile PageTableBitmapPages;
0x0174 0x01E0
LONG volatile PageFileTraceIndex;
0x0178 0x01E8

By itself, the MI_PAGEFILE_TRACES structure at the end, being just 0x50 or 0x78 bytes, would bring the MI_VISIBLE_PARTITION to 0x01C8 or 0x0260 bytes. How, exactly, the MI_VISIBLE_PARTITION ends up so very much larger does not seem to be knowable from the type information in the symbol file.