grammaticality - "bigger" vs. "more big"

As we know, comparatives compare two things. So, for example, we say that one thing is larger or more temperate than another thing.

Now, let us consider the following examples.

A. The African elephant has bigger ears than the Indian elephant.

B. The African elephant has more big ears than the Indian elephant.

If A is "standard" English, why isn't B acceptable English? Is there a grammatical rule that impedes the more big comparative in B?

If this rule exists, why doesn't that apply to "He is more temperate than Carlo, and he chooses words carefully."—which is "standard" and acceptable English?

Answer

@J.R. is absolutely correct, and has provided an excellent example of the kind of ambiguity that can result when basic comparative adjective grammar rules are not followed. But the basic, teachable, and, in an EFL/ESL context, extremely relevant reason that bigger is correct, and more big is ungrammatical as a comparative adjective is because those rules have not been observed. The rules for forming comparative adjectives are fairly straightforward: (1) for one-syllable adjectives, add -er, (2) for two-syllable adjectives ending in -y, change the -y to -i and add -er, and (3) for two-syllable adjectives not ending in -y and all three-or-more-syllable adjectives, use the form "more + adjective".

There are exceptions to these rules, such as fun, the comparative of which is more fun even though it is only one syllable. Another example is often, which is as commonly rendered comparatively using oftener as it is more often. But @Carlo_R was asking for the grammar rules, and I have summarized them.