(Side note: Munhumutapa/Mwenemutapa, or ruler of Mutapa, is the title of the monarch--so we usually just refer to "Mutapa" for the state itself--one that persisted in rump form all the way into the late 1880s under weak Portuguese overrule if one accepts it as the heir to the original Mutapa.)

So, Mutapa was never that size, and none of its direct tributaries were right on the coast. However, because it was the primary trading partner of the Kilwa network on the Swahili Coast and the source of precious metals, ivory, and some finished goods, as well as some captives (many more later on), its importance was easy to magnify. If that's all you know about in the interior for certain, everything becomes it, and the Portuguese learned a fair bit about Mutapa when they came into sustained contact with it in the mid-1500s. Their Eurafrican and acclimated-European envoys clashed with Muslim ones at court when the former arrived, and the Portuguese at Quelimane and Sofala engaged in outright warfare using their coastal alliances against the clients of the Mwenemutapa. Most of these invasions were utter failures early on, but they did eventually begin to shift Mutapan tributaries to themselves, and ultimately coax the court of Mutapa to adopt Christianity in the second half of the 17th century. At that point they began to prise the state apart and establish the offices of the prazeros in the 1660s throughout the kingdom, but that's decades after this map's era.

But in the short term, because the interior wasn't well known sociopolitically (or topographically), it was easy to extend the (unknown) far reaches of Mutapa all the way to the edge of what was known elsewhere. Areas in the far south, where Khoesan people and other south Nguni groups had states, were easy to recast as affiliates of Mutapa in this light, but more to the point the map itself is distorted and confused in ways that expand features associated with Mutapa and its immediate tributaries/neighbors--that river going out to sea roughly where Durban might be expected (Terra do Natal)? There is no such river that winds deep into the interior. The Zambesi isn't horribly off, but the Limpopo is awful, as though the whole southern reach was omitted (as it may have been). On the coast, Mossel Bay and Algoa Bay are remarkably close to their relative positions, as is St. Lucia Bay (north of old kwaZulu today) though shifted a bit east, and the states around Bokoni and the various Thembu and south Nguni kingdoms of the time are collapsed because trade did not carry through them. Naturally, envoys from Mutapa, or those in the court of the Mwenemutapa, would make expansive claims of their reach--claims that the Portuguese had no ability to verify, and that others would accept and portray following the Portuguese. So the distortion of the interior topography renders those details more topological or relational than anything physically reliable, and the mapmaker could expand those features to fill the available area even though no Mwene would claim their power extended clear to another sea. Being conservative with the data doesn't play so nicely with the more physically reliable points on the coast, resulting in the odd visual expansion of Mutapa. [edit: bear in mind, too, that the Dutch didn't have primacy beyond this coast, so their information is compiled from available sources and maps, often by mapmakers who never left Europe; some simply copied others' maps and made small changes to complete their own atlases.]

They've got some of the geographical information uniformly "off," too; things better attached to the Zambesi are stuck on the Limpopo-analogue, because interior charting was entirely by word-of-mouth from Africans and a handful of early merchant travelers to the Portuguese, then through another set of transfers entirely to the Dutch. The town of "Monomotapa" is probably actually the site of Mapungubwe, the former trading depot [edit: that declined with the rise of Great Zimbabwe in the late 1100s/early 1200s but was still a known site in the network]. The center of Mutapa was far to the north, not just inland from the nascent trade stop of Lourenco Marques (Delagoa Bay); Khami, the capital of Butua Torwa, is another possibility, but the accounts the cartographer drew from may have been quite bad; there's a town titled "Butua" that's in a much more correct position relative to the rivers. [I also can't find Ingombe Ilede, the major transshipment point on the Zambesi, anywhere.]

Another interesting thing on the map to me is that the area label "Butua" appears astride 50 degrees longitude [sic; the meridian hadn't been settled by this point], well within the label "Monomotapa," but it was a totally different state and would become quite powerful and dangerous after takeover by the Changamire Rozwi, who eventually vassalized Mutapa around 1700. That label is also located equidistantly between "Monomotapa" the incorrectly placed town and "Zimbaoe" (Great Zimbabwe). A lot of this information about the interior, based on spelling, may come from Duarte Barbosa's early 1500s journals of the Indian Ocean coast and the information he collected from African travelers to Swahili towns.

All that rambling aside, it's a terrible map for the interior, but it bears the stamp of a presumption that the Mutapa state imputed to Portuguese agents, and that they or the cartographer may have expanded further to avoid unattractive blank spots. There is no Cape Town as yet (the VOC expedition goes out in 1651, founding in 1652), so the coast is all that's really known aside from what the Portuguese have encountered. The real Mutapa state, however, had its core and tributaries in the northern half of what is today Zimbabwe, reaching into the areas of Uteve and north of that which today are in Mozambique. Hard boundaries were not really applicable, but as a militarized state, it was at least centralized. They were seen as formidable and powerful because conquest evaded Portuguese attempts in the 1560s and 1570s, and expansive because the relationship between the two was ongoing and the single most important / lucrative one between the European seafarers and the African interior in the region. The map reflects perception through various filters, and so is an interesting testament even if it's totally inaccurate physically.

E. A. Alpers's ancient Ivory and Slaves in East Africa (still remarkably solid, though the conclusions are dated) is basic reading on this, but John Laband's Bringers of War (2011?) also deals with the Portuguese in the general region. T. H. Elkiss's Quest for an African Eldorado is another old, but readable, text on the specific relationship, and the general political organization of the interior states is Innocent Pikirayi's bread and butter (The Zimbabwe Culture, 2001, and many journal articles since). There may be a few other sources but that's what I have on the tip of my brain.

[edits: bad sentences, uncertain referents, map note, other artifacts of writing while full of cold medicine; clarifications hither and yon]