kā is the substitute of ku when the meaning is 'a small'.,
As कामधुरम्, कालवणम्, काम्लम् ॥ Though the second member may begin with a vowel, yet this substitution takes place, in spite of 6.3.101 : as कोष्णम् ॥,
ईषदर्थे च
Adhyāya 6 · Pāda 3 · Rule 105