If the ratio of threads is the same, then it's the same tartan, just different threads. Is the weight of the cloth less? That could also shrink the pattern. You'll get better answers from others, but perhaps this will get you started.
Now, work on that beer belly! 
Rev'd Father Bill White: Mostly retired Parish Priest & former Elementary Headmaster. Lover of God, dogs, most people, joy, tradition, humour & clarity. Legion Padre, theologian, teacher, philosopher, linguist, encourager of hearts & souls & a firm believer in dignity, decency, & duty. A proud Canadian Sinclair with solid Welsh and other heritage.
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