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Problem

I do not understand why larger $p$ will give a larger covering number.

Since when $p\geq q$, the corresponding hypercube is also larger (by $\| x \| _ { q } \leq n ^ { ( 1 / q - 1 / p ) } \| x \| _ { p }$ when $p > q>0$) and thus should give smaller covering set. Covering number is essentially the minimum cardinality of covering set, so it seems that larger $p$ should give smaller covering number?

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Since you've tagged machine learning, I'll address this setting. Indeed, for the usual $\ell_p$ norms on $\mathbb{R}^n$, we have $||x||_p\le ||x||_q$ for $p>q$. However, in machine learning applications, these are normalized norms (see, e.g., Mendelson, A few notes on Statistical Learning Theory https://people.eecs.berkeley.edu/~jordan/courses/281B-spring04/readings/mendelson.ps ). Let $D$ be a distribution on $\{1,\ldots,n\}$. Then the corresponding $\ell_p(D)$ norm is defined by $$ ||x||_{\ell_p(D)}^p = \sum_{i=1}^n D(i)|x_i|^p. $$ You can verify that for these norms, the inequality goes the other way: $||x||_{\ell_p(D)}\ge ||x||_{\ell_q(D)}$ for $p>q$. This explains the covering number inequality as well.

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