Hrm... my two sources of data for the Traveller galaxy structure don't quite equate, leading to fairly dramatic mismatches.
If you measure the Local Arm's girth at the Ray 0 i.e. through Reference (Core 0140) on the detailed map from Security Leak by Mike Mikesh, you get a distance of 1600 parsecs. If you measure the distance on the detailed map by Clifford Linehan - which he used to construct the Zhodani Core Route - you get a distance of 1760 parsecs.
That's about a 10% difference... and it's going to bother me. I dug a bit further and it looks like the Security Leak #5 map is the original source, per this conversation between Marc Miller and Clifford Linehan. Mr. Linehan took Mike Mikesh's original, scanned it, cleaned it up, and handed the results back to MWM who took it under the FFE fold. This would indicate that either Mr. Linehan corrected the size (to fit known dimensions of the galaxy) or accidentally scaled it up. (Charted Space doesn't quite line up either.)
Since Traveller parsecs are based on a hexagonal grid, it's actually easy to introduce an error of around 14% - hexes pack more tightly horizontally than vertically, with a ratio around 0.868. This is factored in when I map from image-space to world-space (and you'll see residue in the API). Here's a sample - the red square is precisely 10 parsecs tall, but more than 11 parsecs wide - yet it's a perfect square if you count the pixels.
If I had to guess, I'd say that one of the two galactic-scale map authors was unaware of this, and and used a horizontally-biased parsec scale for determining the number of sectors between Charted Space and the galactic core.