Translation:This is the fattest lion that I have ever seen.
wouldn't that be singa ini tergemuk.... the statement focuses on 'this' rather than 'this lion'. I don't think it changes the sentence in any meaningful way but I think differences like that are important for us when we are learning so that we can construct sentences with a good understanding of exactly what they mean
Thank you. You said it succinctly. The two sentences mean essentially the same thing but they use a different structure to mean it. The relationship between meaning and structure is what we're trying to learn. The distinction between the two sentences may be irrelevant in a conversation about lions but it's critical to us.
For example, a student still wondering if "ini" might sometimes come before the noun (I believe it cannot), or if "tergemuk" might head a noun phrase (I believe "yang" would be needed), could be led astray by the acceptance of "this lion is the fattest ...".
Furthermore, the difference in focus does matter in skillful conversation. To explain a blurry photo of a brownish blob I would far more likely say "This is the fattest lion ..." than "This lion is the fattest".