In wikipedia it says that the Kitara Empire included Uganda, northern Tanzania, Malawi, Zambia, Rwanda and Burundi, who is quite big, but this article doesn't have any citations and I didn't found too much in other sites.
Off the bat, that claim looks extremely suspicious. The failure to provide any citation for such a claim is also worrisome.
In Kingdoms of Africa by Saheed Aderinto, the entry for Kitara states:
....At its height, around the 16th century, the Kitara Empire encompassed present-day Uganda, parts of western Tanzania and Kenya, the eastern Democratic Republic of Congo, most of Rwanda, and parts of northern Burundi...
pointedly, it does not mention Malawi or Zambia.
Christopher Ehret discusses Kitara and their Bunyoro successor state in Civilizations of Africa 2nd edition. He provides maps which estimate the sphere of influence of Kitara from 1100-1250 (map 17) and circa 1400 (map 18), and sphere of influence of Bunyoro circa 1550 (maps 21 and 22). In text and in maps, Ehret depicts Kitara and Bunyoro as territorrially equivalent to modern Uganda, Rwanda, Burundi and kagera province of Tanzania.
David Lee Schoenbrun discusses political centralization up to 1400 in A Green Place, A Good Place. He mentions oral traditions concerning the Cwezi dynasty, and paints a picture of political centralization and regional competition in the period from 1250-1400. Like Aderinto, there are no claims made about Cwezi influence stretching all the way to Malawi or Zambia.
So, there seems to be a general consensus along Aderinto and Ehret's characterization that Kitara and later Bunyoro were a bit larger than Uganda. No reason to believe it stretched all the way to Malawi.