Sorry, I got it slightly wrong.
ipfs object operates on the merkledag-pb layer, which wraps the unixfs-pb.
1677 is the actual file size as reported by a field in the unixfs-protobuf (which ls uses).
1685 is the DataSize in the block, which is the [full_blocksize - links size] (the full merkledag-pb block being 1688 ytes). So this is the size of the unixfs protobuf inside the pb-merkledag block.
Instead, if we want answers from the unixfs layer we need to use ipfs files:
ipfs files stat /ipfs/bafybeibxm2nsadl3fnxv2sxcxmxaco2jl53wpeorjdzidjwf5aqdg7wa6u
bafybeibxm2nsadl3fnxv2sxcxmxaco2jl53wpeorjdzidjwf5aqdg7wa6u
Size: 0
CumulativeSize: 6544
ChildBlocks: 7
Type: directory
ipfs files stat /ipfs/bafybeibxm2nsadl3fnxv2sxcxmxaco2jl53wpeorjdzidjwf5aqdg7wa6u/about
QmZTR5bcpQD7cFgTorqxZDYaew1Wqgfbd2ud9QqGPAkK2V
Size: 1677
CumulativeSize: 1688
ChildBlocks: 0
Type: file
In the last one, Size is the actual file size as it would be on disk. CumulativeSize corresponds to that of ipfs object stat, that is, the actual size the dag under that CID takes on IPFS.