Most often “и” translates to “and”, but it's used in other ways as well. For example, it can be used before each item in an enumeration. Compare:
- “Я купил чай, хлеб, сыр и чеснок”
- “Я купил и чай, и хлеб, и сыр, и чеснок”
The first sentence sounds dry, a mere statement of fact: “I bought tea, bread, cheese, and garlic”. In the second sentence, by prefixing each item with “и”, the speaker highlights the exhaustiveness or extensiveness of the enumeration, as in “I bought so many things” or “I bought all the things we needed”.
When paired with other words, “и” can add emotional depth and subtext. Let's take a look at your examples:
“Но я очень люблю мучные изделия, поэтому и расту как на дрожжах.” Here “и” strengthens “поэтому”, highlighting that the consequent is obvious. You could translate it as “naturally”: “But I love baked goods, so naturally I grow by leaps and bounds”. It could be omitted without loss of meaning, only the tone of the text is affected.
“Я тебя и такую люблю”. Here the use of “и” means that the stated information is a surprising instance of a more general rule, much like “даже”. The sentence translates to “I love you even when you are like this”. A few similar examples:
- “Мне и без соли вкусно” (I like the taste even without salt).
- “Они и кошек дрессируют” (They train even cats)
“но и” translates to “but also”, and I would typically expect it to be used with “не только” (“not only”). For example, one could say “Она не только умна, но и трудолюбива” (She is not only smart but also industrious).