As requested, Rapid Resizer Online now has options to keep lines smooth and thin when enlarging patterns or any kind of line drawing. Your enlargement's lines won't get that jagged blurry stair step shape.
It's accessible as the "Lines" option on the page where you enter your design's new size.
There's some distortion. Less with the "smooth" option than with "smooth & thin".
Please try it and let me know what you think.