Yo riego las plantas con una regadera pequeña, porque la grande pesa demasiado.

That’s right: in Spanish, subject pronouns are often omitted because the verb ending already shows who is doing the action.

• (Yo) riego = I water

So yo is optional here.
The sentence would still be correct as:

• Riego las plantas con una regadera pequeña...

Including yo can add:

• emphasis: I water the plants...
• contrast: I use the small watering can, not someone else

So this yo is not required, but it is perfectly natural.

Riego is the first person singular present indicative form of regar (to water).

The present tense forms are:

• yo riego
• tú riegas
• él/ella riega
• nosotros regamos
• vosotros regáis
• ellos/ellas riegan

This verb is stem-changing in the present tense: e → ie in most forms.

So:

• regar → riego
• but nosotros regamos, not riegamos

In this sentence, riego most naturally means a habitual action: I water the plants.

Spanish uses articles more often than English does.

Here, las plantas means the plants, and it sounds natural because the speaker is talking about a specific set of plants, probably the ones at home or in the garden.

English often says:

• I water plants or
• I water the plants

Spanish is more likely to use the article here:

• Riego las plantas

Saying riego plantas is possible in some contexts, but it sounds more general and less natural in this everyday sentence.

Here con means with, in the sense of using something as a tool.

So:

• con una regadera pequeña = with a small watering can

It does not mean together with in this sentence.
The context makes it clear that the watering can is the instrument being used.

In Spanish, descriptive adjectives usually come after the noun.

So the most neutral, ordinary order is:

• una regadera pequeña = a small watering can

You can sometimes put the adjective before the noun, but that often adds a different tone, such as:

• a more literary style
• emphasis
• a more subjective feeling

In this sentence, regadera pequeña is the normal choice because it is just a simple physical description.

Spanish often avoids repeating a noun when it is already clear from context.

So:

• la grande = the big one

Here, la stands in for la regadera, and grande describes it.

This is very common in Spanish:

• Quiero la roja. = I want the red one.
• Prefiero el pequeño. = I prefer the small one.

Because regadera is feminine singular, the article is la.

Because the omitted noun is regadera, and regadera is feminine.

So even when the noun is left out, the article still has to match it:

• la regadera grande
• la grande

Notice:

• pequeña changes form to match the feminine noun
• grande stays the same in singular, but the article la still shows feminine gender

They are not the same.

• la grande = the big one
• la más grande = the biggest one

So in your sentence:

• la grande pesa demasiado means the big one is too heavy

It does not necessarily mean it is the biggest of all possible watering cans. It just contrasts with the small one.

Because porque as one word means because.

This is one of the most common distinctions learners need:

• porque = because
• por qué = why / for what reason

Examples:

• No voy porque estoy cansado. = I’m not going because I’m tired.
• ¿Por qué no vas? = Why aren’t you going?

In your sentence, it gives the reason:

• porque la grande pesa demasiado

That comma is possible because the second part is being added as an explanation.

• Yo riego las plantas con una regadera pequeña, porque la grande pesa demasiado.

In Spanish, porque clauses often appear without a comma, especially when they are tightly connected to the main clause. But a comma can be used when the speaker presents the second part more as an explanatory comment.

So here the comma is acceptable and natural.
You may also see the sentence written without it in other contexts.

Literally, pesa demasiado means:

• it weighs too much

In natural English, that usually becomes:

• it’s too heavy

Here:

• pesa = weighs
• demasiado = too much / excessively

So the idea is not just that the big watering can is heavy, but that it is heavier than is comfortable or practical.

Yes, but the meaning changes slightly.

• pesa demasiado = it weighs too much / it’s too heavy
• es muy pesada = it is very heavy

The first one suggests excess: it is heavier than the speaker wants or can easily manage.
The second only says that it is very heavy, without necessarily stressing that it is too heavy.

So pesa demasiado is a better match here because it explains why the speaker uses the small watering can.

Yes, in this sentence it clearly means watering can.

The context makes it unmistakable:

• riego las plantas = I water the plants
• so regadera is the tool used for that

Also, regadera can have other meanings in some varieties of Spanish, but here the meaning is definitely watering can.