Его доля была маленькой.

Because доля is a feminine singular noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• был = masculine
• была = feminine
• было = neuter
• были = plural

So:

• доля была = the share was
• not доля был, because доля is feminine.

After быть in the past tense, Russian often uses the instrumental case for the predicate adjective.

So:

• маленькая = nominative feminine singular
• маленькой = instrumental feminine singular

That is why the sentence says:

• Его доля была маленькой.

This structure is very common in Russian.

You may also hear Его доля была маленькая, and that is possible in some contexts, but маленькой is a very standard and natural choice after была.

It is the instrumental singular feminine form of маленький.

Here is the adjective in a few forms:

• masculine nominative: маленький
• feminine nominative: маленькая
• feminine instrumental: маленькой

Since доля is feminine singular, the adjective must match it in gender and number, and because of the construction with была, it appears in the instrumental.

Yes, его can mean both him and his, depending on the context.

In this sentence, его is a possessive pronoun meaning his:

• его доля = his share

You can tell it is possessive because it directly modifies the noun доля.

Compare:

• Я вижу его. = I see him.
• Я вижу его дом. = I see his house.

So in Его доля была маленькой, it clearly means his.

As a possessive pronoun, его is indeclinable in modern Russian in this use.

So you say:

• его доля = his share
• его дом = his house
• его друзья = his friends

It does not change like adjectives do.

That is different from words like мой, твой, наш, which do change:

• моя доля
• мой дом
• мои друзья

But его stays его.

This is a very common pronunciation question.

Although it is written его, in standard pronunciation it is usually pronounced roughly like yevo.

This happens because in forms ending in -ого / -его, the г is often pronounced like в in this type of word.

So:

• written: его
• pronounced: approximately yeVO

This is normal and standard.

Доля can mean several related things depending on context:

• share
• portion
• part
• sometimes even lot / fate

In this sentence, it most naturally means something like:

• share
• portion

So Его доля была маленькой most likely means His share was small or His portion was small.

Yes. Russian word order is more flexible than English word order.

The neutral order here is:

• Его доля была маленькой.

You could also encounter something like:

• Доля его была маленькой.

But that sounds more marked, literary, or emphatic.

For a learner, Его доля была маленькой is the most neutral and natural version.

Yes. In the present tense, Russian usually omits the verb to be.

So:

• past: Его доля была маленькой. = His share was small.
• present: Его доля маленькая. = His share is small.

Notice two things:

1. была disappears in the present tense.
2. The adjective is usually маленькая in the present-tense sentence.

A simple approximation is:

yi-VO DO-lya by-LA MA-lin'-koy

A few stress points:

• его → stress on the last syllable
• доля → stress on до
• была → stress on the last syllable: была́
• маленькой → stress on the first syllable: ма́ленькой

A more natural pronunciation would sound roughly like:

yivo DOLya byLA MAlen'koy

Russian has no articles.

So доля can mean:

• a share
• the share
• sometimes just share

The exact meaning depends on context.

That is why Его доля была маленькой can be understood as:

• His share was small
• His portion was small

without any separate word for the or a.