|This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)
A diminutive is a variation of a word that, if the if the object is tactile, conveys smallness of size.
If not tactile, and in general, it may be used for things like endearment, softness, intimacy, condenscendance, or names for comparable concepts (makin a cottage out of a word meaning house, making warm/lukewarm from hot).
They are usually created with affixes, often suffixes.
An augmentative is the opposite: an intensification or augmentation of some sense of a word. The sense can be size, but also just strengthen some quality, such as dislike, or most comparatives in the superlative sort of sense.
These are also usually created with an affix and then regularly a suffix, but various languages (additionally or primary) use prefixes.
In various languages, diminutives and/or augmentatives may appear as a small set of cases and according clear morphological rules, and/or as exceptions not following any general rules. Dutch can generally apply a diminutive suffix, but has no augmentative other than for cases like 'grandfather'.