No, I'm afraid you can't just omit the articles. If you're talking about the fruits themselves, the words 'watermelon' and 'apple' are singular, countable nouns. Countable nouns are 'things', like apples. Uncountable nouns are 'stuff', like juice. An important rule for you to remember is that singular countable nouns need to have an article. You have two main options: A watermelon is much bigger than an apple. [singular + indefinite article = countable e.g. a fruit] or Watermelons are much bigger than apples. [plural, with zero articles] If you just say 'apple' or 'watermelon', in the singular with no article, this means that you are talking about something uncountable connected with these fruits - perhaps their juice or their flavour. For example: What fruit juices do you have? We've got apple and watermelon. [ singular, no article = uncountable e..g 'apple [juice]'] What flavour ice cream would you like? Watermelon or apple? I'll have watermelon, please. Watermelon is tastier than apple. [ singular, no article = uncountable e..g 'apple flavour'] Now, a question for you, given that your English level is quite high: Why exactly did you think it might be OK to omit the article? Was there a rule about articles which you had learnt and which you thought might apply here?

You can say "a watermelon is much bigger than an apple." You can say "watermelons are bigger than apples." Now, here is a tricky one. You can say any of these things: "A pineapple is sweeter than a watermelon." "Pineapples are sweeter than watermelons." "Pineapple is sweeter than watermelon." We can do this for two reasons. The first, and most important, is that "sweet" is an adjective that makes sense with an uncountable noun. "Big" does not make sense with an uncountable noun. The second is that for some foods, the same word can be used as a countable noun, meaning "a natural-sized piece of the food." It can also be used as an uncountable noun, meaning "the flesh, the substance, the edible part, the stuff inside." In fact, watermelon is sweet, watery, and red. Yet, a watermelon is big, heavy, and green. It is fairly common to use "pineapple" and "watermelon" as uncountable nouns, referring to the yellow stuff or the red stuff inside the fruit; whereas it is wrong--or at least very unusual--to use "apple" that way. I would not say "apple is sweet and crunchy," I would say "the flesh of the apple is sweet and crunchy."

Watermelons are much bigger than apples.