It appears a bug was introduced in version 6.2:
Project created/saved in 6.1 and opened with 6.2:
With the 1200px desktop-only layout, a Gallery item with the “Stretch > FIT” option displays images with a very high zoom level, well beyond the area’s aspect ratio constraints.
The problem is visible in both the preview and the published page.
If the original project is reopened with version 6.1, the problem disappears.