Learn Portuguese (Portugal): A memória deste modelo é maior.

Just like in English where we say "bigger" instead of "more big," Portuguese has irregular forms for some comparatives. The word maior is the irregular comparative form of grande (big). You should always use maior rather than mais grande.

Because memória is a feminine word, shouldn't maior have a feminine ending?

No, maior does not change for gender. It is exactly the same for both masculine and feminine nouns. It only changes for plural words, where it gets an "es" at the end to become maiores. Since we are talking about one single memory here, we just use maior.

Where does the word deste come from in this sentence?

The word deste is a combination of the preposition de (of) and the masculine singular word for "this" (este). Instead of saying de este, the words merge together. Literally, the sentence translates to "The memory of this model is bigger."

Why use é (from the verb ser) instead of está (from estar)?

We use é because the memory capacity is a permanent, built-in characteristic of this specific model. While technology changes over time, the physical design of this exact model does not change, so we use the permanent "to be" verb ser.