とても・かなり・ずいぶん

かなり is particularly well suited to presenting a degree as substantial in relation to a norm or implicit threshold without necessarily foregrounding the speaker's personal surprise. This is why it also occurs naturally in informative, quantitative, and relatively analytical descriptions.

ずいぶん is especially natural when a quality or change appears remarkable in relation to what the speaker expected. Here, it conveys not only that the child has grown a great deal, but also that the extent of the change strikes the speaker.

かなり regularly modifies nouns through の, as in かなりの時間 and かなりの人数. ずいぶん does not normally form the equivalent neutral construction with の; in the attributive pattern ずいぶんな～, it readily takes on a more marked evaluative meaning.

ずいぶん is already a complete adverb. In ずいぶんと, と can make the connection with what follows more explicit and may affect the rhythm or emphasis of the sentence, but it does not introduce a regular semantic difference in degree.

ずいぶん can naturally express the substantial extent of an action, as in ずいぶん食べた or ずいぶん待たされた. とても, by contrast, is not normally used directly with an affirmative verb simply to mean that someone did a lot of something.

In the attributive construction ずいぶんな～, ずいぶん can readily acquire a strongly evaluative, ironic, or critical nuance. This differs from the adverbial ずいぶんと失礼だ, where the expression functions adverbially and means something like "really very rude."

In some contexts とても may be perceived as the stronger intensifier, but this should not be turned into a rigid hierarchy. The key feature of かなり is that it presents a degree as noteworthy in relation to an implicit parameter, norm, or threshold.

かなり is highly versatile in informative and written contexts because it can present a noteworthy degree while maintaining a relatively detached tone. ずいぶん, by contrast, tends to make the speaker's or writer's personal impression more noticeable.

とても is the most neutral choice for simply presenting a high degree; かなり indicates that the degree is substantial in relation to a reference point or norm; and ずいぶん makes the departure from expectations and the speaker's subjective reaction more apparent.