Malcolm,
Yep, when you enlarge the design, you are making the column stitching wider that your machine will allow. You can either do a tatami (regular ) fill, or, depending on your software, do a column stitch that doesn't run the full width but has a stitch in the middle, effectively making it one column split into two parts. What you are running into is common for beginning embroiderers. We all believed we can take a smaller design and just enlarge it, but it doesn't work that way. Embroidery has limitations and knowing what they are and how to work around them are what it's all about

For large lettering, I think most of us would use applique to cut down on the stitch count, but sometimes you have to do what the customer prefers and what he will pay for.