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They don't, the Ash Mountains are just west-eastern line and the north-south line which is is Ephel Dúath.
The highest mountains in Middle-Earth were Ered Engrin in the First Age, and if the Iron Hills and Grey Mountains were originally a part of them (as it is often speculated, and it is very much possible), they'd be possibly also the largest in the terms of length. However, after the destruction of Beleriand, concerning the length of the mountain range, the Hithaeglir still wound't be the greatest. They were about 700 miles long from north to south, including the mountains of Angmar in the measurement, but the White Mountains were larger in that point - according to Karen Wynn Fonstad's Atlas, they were 852 miles long. And the Ered Luin in the First Age, before the destruction of Beleriand, were more than 900 miles long, thus larger than Hithaeglir even if you didn't count Ered Engrin.
Of course you asked about Middle-Earth, by which I do not mean Aman - otherwise it'd be Pelóri.
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"Should the story say 'he ate bread,' the dramatic producer can only show 'a piece of bread' according to his taste or fancy, but the hearer of the story will think of bread in general and picture it in some form of his own." -On Fairy-Stories
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