The needle size and thread size shown in the photo above are identical. The green yarn shows plain stocking/stockinette stitch worked with the same number of stitches as the patterned knitting below.
Patterns for Nordic and Fair Isle colour work (and cable) knitting often have a lot more stitches than a pattern for a similar garment worked in plain stocking/stockinette stitch.
This means that if the pattern is designed for colour work but knitted in plain stocking/stockinette stitch, it will often come out much wider than expected. The travelling yarns tend to pull the fabric of the knitting much closer together and make a more dense finished item.