I don't think there's much more to it: some adjectives can simply be used as adverbs too. Some can only be so used in certain idiomatic expressions (your estimate fell short), others in a broader context (she drove by fast). There are also other adverbs that simply don't end in -ly, like soon and yonder.
The now productive suffix -ly for adverbs is relatively recent; Dutch and German don't have it—that is, the suffix exists, but it is used differently. In Proto-Germanic/Gothic, the suffix -lîko- could be used to form adjectives from nouns and other adjectives. (Cf. manly, soldierly, womanly, masterly.) The normal suffix for adverbs was -e in Old English, which still exists in German. When the -e ceased to be pronounced in English, at some point -ly became the normal adverbial suffix (it is supposed that there had been adverbs in -ly that had the suffix because they were based on earlier adjectives with -ly, Oxford English Dictionary on -ly_2). This history of the suffix is probably the reason why we still do not use it consistently today.
A small list of seemingly normal English adjectives that can also be adverbs, to which I invite anyone to add more examples:
- Rest easy.
- Work hard.
- Sit still.
- Fall short.
Edited: The question remains why these adverbs cannot be used before the finite verb:
They quickly followed her.
They soon found her.
*They fast drove to the palace.
I have a theory: because words like fast can be used as regular adjectives, and because many verbs can be used as nouns, it would be very confusing if we could say both *they hard work and their hard work. It could easily lead readers on a false scent, especially in more complex sentences. That could be a reason why we do not use these words in that particular position.
Actually the most common phrasing I've personally seen is:
It is high in calories.
The number of calories can be thought of as either a number (number of) or aggregate (amount of). Other answers/comments have indicated that there's debate over which is more common/appropriate, but what one you use influences which adjective you pick.
Per this NGram:
large number of --> Most Common
large amount of --> Common
high number of --> Uncommon
high amount of --> Almost Unheard Of
Best Answer
Such an interesting question! Such thought-provoking remarks in the comments beneath the question!
Such comments make me wonder, what is the best way to answer such a query? Maybe a few things need to be addressed first, such as:
A good online dictionary, such as Wordnik, might help. There, we can find such a great number of meanings and examples, that it will become readily apparent that such a question is not so easily answered. English can be such a hard thing to fully grasp, particularly when dealing with such words – words that can be used in such a myriad of ways! After all, many words, though but one word, can be used as different parts of speech: adjectives, adverbs, pronouns and such; such is one such word.
Really, Ivan, you should go visit such a website, and behold the diverse ways such a simple word can be used. Such a curious man is bound to learn something from such a great smattering of examples!
which is to say, there's no EASY way to tell when to use "such", and when to use "such a" - the word is simply too flexible to nail down in such a fashion...