Denny and Creider (1986 :223) state that the `primary meaning [of Proto-Bantu Class 7] is instrumental artifact'. If one interprets `primary meaning' as `prototypical meaning', I think this is right, and that it is still true of Swahili. I have added to this the specification `small enough to hold in the hand', because this applies to the majority of terms for instrumental artifacts in Class 7, and it provides a motivation for the major semantic extension within this class, to `small entities in general', not all of which are instrumental artifacts.
Among `small entities' there are several subgroups, most of which are self-explanatory. I will comment on the ones that are less obvious. First, the category `pieces/parts of things'. Parts or subdivisions of things are smaller than the whole, so this category includes both reference to size and an implicit comparison between the part and the whole (recall Zawawi's 1979:115 definition of this class as `comparison of size or manner'). This part-whole comparison is carried over into a further extension, to `shortened things', that is things that have been truncated through being worn down or cut, and to terms for people with physical defects, conceived as not-whole. These latter terms generally have derogatory connotations in Swahili which, not coincidentally, has a single adjective for `whole', `healthy', and `adult/mature', i.e. -zima. As pointed out by Denny and Creider (ibid.), `it is a fairly natural extension from `used object' to `despised object''. I am arguing here that the metaphor of size plays a role in this extension in Swahili.
The salient characteristic of terms in the subcategory `pointed things' is that a point or angle occupies a small amount of space. Even if the whole is large, such as a mountain, the pointed part (`peak') is relatively small. In the case of pointed parts, too, there is an implicit comparison between part and whole.
The category `part of substance' is more abstract than the ones just discussed, but its connection to part-whole relations is nonetheless apparent. This category includes terms denoting subdivisions of time and space. Height, depth, and units of measurement divide and delimit potentially extended spaces or spans of time into measurable parts. Here, too, there is an implicit comparison between the measured entity and the undelimited remainder.
Swahili grammars often point out that the prefix ki- is used to derive `adverbs of manner' (cf. Ashton 1944:165; Polome 1967:100), but they make no connection between this `function' and the noun class meaning of ki-. Zawawi's (1979:115) suggestive definition for the Class 7 prefix ki-, `comparison of size or manner', is the first to connect these ideas explicitly, but Zawawi does not explain how they are connected. I believe the link lies in a metaphorical extension of the part-whole relation to qualities or attributes, in a way reminiscent of the English expression `a chip off the old block'. The relationship of similarity can be thought of as a partial overlap in substance between the entities that are regarded as similar. Thus a very sweet banana overlaps in part with the sweetness of sugar; a butterfly that flies slowly partakes, so to speak, in the quality of slowness. The same rationale can be extended to deverbal nouns denoting human beings who habitually perform the action expressed by the verb.
Finally, the terms in the category `ailments associated with body parts' are all connected to their respective body parts by metonymic extension. Although some of them are actually associated with small body parts (`inflammation of the eyelid'), one could easily imagine this category being extended to other body parts via metonymy.
The above categories account for over 90% of the Class 7 nouns in the database. Of the rest, in some cases not enough information is provided in the dictionary gloss to decide whether they fit or not (for example, kilendo `kind of fish'; kilua `kind of sweet smelling flower'-- are they small?). Some may be deverbal nouns or `similarity' nouns whose source is now obsolete (e.g. kisutuo `food received after a task has been completed'; kifabakazi `Nandi flame tree'). Some are loanwords that had ki- as initial syllable, and so may have been placed in this class for phonological reasons.
There is one set of apparent anomalies that deserves additional comment, however. This is a group of terms referring to large, dangerous animals or birds: kifaru `rhinoceros', kiboko `hippopotamus', kingugwa `large spotted hyena', kipungu `eagle', and kipanga `Dickinson's falcon'. These terms are strikingly anomalous: why should large, predatory animals be placed in a class whose most prominent characteristic is small size, often with a connotation of insignificance? Interestingly, three of these terms (`hippopotamus', `rhinoceros', and `eagle') are replacements for terms that were originally in Class 9 (the `animal' class) in Proto-Sabaki (N + H 296). In other words, these three animals were moved from the `animal' class to Class 7 (kipanga `Dickinson's falcon' was already in Class 7 in PSA, and N + H do not give a reconstructed form for `spotted hyena'). One possible explanation is that these terms started out as euphemisms. Putting names of large, dangerous animals in the class of small, manipulable things could be a way of figuratively neutralizing or diminishing their power.