Actually the sentence "this office is big" is definite (i.e. the office is defined, by virtue of using "this") and hence in Arabic it would be: هذا المكتب كبير (al-maktab, the office). Thus, for the Arabic sentence above (where maktab is indefinite), the translation has to be "this is a big office"
Because the office is defined by stating this office, should it not at least in its translation be stated, This office is big? In English both convey the same notion.