Chumba cha watu wawili ni kikubwa kuliko chumba cha mtu mmoja.

Because cha links the two nouns and means something like of / for / belonging to.

So:

• chumba = room
• cha = of (agreeing with chumba)
• watu wawili = two people

So chumba cha watu wawili literally means room of two people, or more naturally, a room for two people / a double room.

In Swahili, this kind of connector changes form depending on the noun class of the first noun. Since chumba is in the ki-/vi- class, the connector is cha.

Cha is the associative marker that agrees with chumba.

Swahili uses agreement prefixes, so the word meaning of changes depending on the noun class:

• mtu wa watoto = person of children
• kitabu cha mwanafunzi = book of the student
• nyumba ya mwalimu = house of the teacher

Here:

• chumba belongs to the ki-/vi- noun class
• the associative marker for that class is cha

So:

• chumba cha watu wawili = room of two people

If the first noun were different, the connector would change.

Because watu is plural and mtu is singular, and numbers in Swahili often agree with the noun class.

• mtu = person
• watu = people

For people, the numbers one to five usually take agreement forms:

• mtu mmoja = one person
• watu wawili = two people
• watu watatu = three people
• watu wanne = four people
• watu watano = five people

So:

• mmoja agrees with singular mtu
• wawili agrees with plural watu

This is why you do not say watu mbili for two people.

Both relate to two, but they are used with different noun classes.

• wawili is used with people
• mbili is used with many non-human nouns

Examples:

• watu wawili = two people
• vitabu viwili = two books
• nyumba mbili = two houses

So in your sentence:

• watu wawili is correct because we are talking about people

Using mbili with watu would sound wrong.

Here ni works like is / are.

So:

• chumba ... ni kikubwa = the room ... is big

In equational or descriptive sentences, Swahili often uses ni before a noun or adjective phrase:

• Yeye ni mwalimu = He/She is a teacher
• Kitabu ni kizuri = The book is good

In your sentence:

• ni kikubwa kuliko... = is bigger than...

So ni is the basic copula, similar to English is.

Because adjectives in Swahili agree with the noun they describe.

The noun here is chumba, which is in the ki-/vi- noun class. So the adjective also takes the ki- agreement prefix.

• -kubwa = big
• kikubwa = big, agreeing with a ki- class noun

So:

• chumba kikubwa = a big room

If the noun were plural, it would change:

• vyumba vikubwa = big rooms

This agreement is a very important part of Swahili grammar.

Yes, kikubwa by itself means big. The comparative meaning comes from kuliko.

So the pattern is:

• [noun] ni [adjective] kuliko [noun]
• literally: [noun] is big compared with [noun]
• natural English: [noun] is bigger than [noun]

Examples:

• Juma ni mrefu kuliko Asha = Juma is taller than Asha
• Nyumba hii ni kubwa kuliko ile = This house is bigger than that one

So in your sentence:

• ni kikubwa kuliko... = is bigger than...

Swahili does not usually create comparatives by changing the adjective itself the way English does with big → bigger.

Kuliko means than in comparisons.

It links the two things being compared:

• A ni kubwa kuliko B = A is bigger than B
• A ni nzuri kuliko B = A is better/nicer than B

In your sentence:

• Chumba cha watu wawili ni kikubwa kuliko chumba cha mtu mmoja
• A double room is bigger than a single room

So kuliko is the comparison word that corresponds to English than.

In Swahili, the noun usually comes before the number or adjective.

So the normal order is:

• watu wawili = two people
• mtu mmoja = one person
• chumba kikubwa = big room

This is different from English, where numbers come before nouns:

• English: two people
• Swahili: watu wawili

So watu wawili is the natural Swahili word order.

Because Swahili often expresses this idea descriptively.

• chumba cha mtu mmoja = room for one person
• chumba cha watu wawili = room for two people

This is a very natural and common way to say single room and double room in Swahili.

Languages often package ideas differently. English uses a compact adjective like single or double, while Swahili commonly uses a phrase showing for how many people.

Swahili usually does not use articles like English a and the.

So:

• chumba can mean a room or the room, depending on context
• mtu mmoja can mean one person or a single person, depending on context

That means:

• Chumba cha watu wawili ni kikubwa kuliko chumba cha mtu mmoja could be understood as:
• A double room is bigger than a single room or
• The double room is bigger than the single room

The exact choice depends on the situation.

Yes, but it means something slightly different.

• chumba kikubwa = a big room / big room
• chumba ni kikubwa = the room is big

So:

• without ni, it is a noun phrase
• with ni, it becomes a full statement

In your sentence, you need a full statement because you are saying something about the room:

• Chumba cha watu wawili ni kikubwa kuliko...
• The double room is bigger than...

So ni is necessary here.

It can naturally mean double room.

Literally it is:

• room of/for two people

But in real usage, that is exactly how you would describe a double room. Likewise:

• chumba cha mtu mmoja = single room
• chumba cha watu watatu = room for three people

So the phrase is both literal and practical.

If you wanted to talk about rooms instead of a room, several words would change because of noun-class agreement.

Singular:

• Chumba cha watu wawili ni kikubwa kuliko chumba cha mtu mmoja.

Plural:

• Vyumba vya watu wawili ni vikubwa kuliko vyumba vya mtu mmoja.

Changes:

• chumba → vyumba = room → rooms
• cha → vya = of, agreeing with plural vyumba
• kikubwa → vikubwa = big, agreeing with plural vyumba

This shows how strongly Swahili grammar depends on noun-class agreement.