Principal English Translation: 

smallness (a compounding element that must be bound to another element; see Karttunen); this is the singular ending; the plural would be -totontin (see Carochi; see also our entry for -ton.)

Orthographic Variants: 
Frances Karttunen: 

-TŌN-TLI compounding element conveying the sense of smallness, insignificance; pl: -TOTŌNTIN. Although necessarily bound, this behaves in a manner distinct from suffixes and postpositions. It has its own absolutive suffix and forms the plural by reduplication of itself plus addition of the plural suffix -TIN. If the noun with which it compounds does not itself take an absolutive suffix, then the compound does not either, and -TIN is omitted in the plural; CHICHI ‘dog’, CHICHITŌN, [little dog] CHICHITOTŌN ’[little] dogs’.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 247.

Horacio Carochi / English: 

The plural of tōntli is totōntin, as with piltōntli, boy; plural pīpiltotōntin. The plural of tōn is totōn: chichitōn, [miserable] little dog, plural chichitotōn.
Horacio Carochi, S.J., Grammar of the Mexican language with an explanation of its adverbs (1645), translated and edited with commentary by James Lockhart, UCLA Latin American Studies Volume 89 (Stanford: Stanford University Press, UCLA Latin American Center Publications, 2001), 43.

Attestations from sources in Spanish: 

-tontli o -ton = Se puede traducir como -illo en castellano. A veces tiene cierto sentido despectivo. No indica reverencia.
Notas de Rebecca Horn de sus clases en náhuatl con James Lockhart. Stephanie Wood está tomando ejemplos de estas notas para este diccionario.