占める: an entire wall and half of all users
Compare the wall occupied by a bookcase with the proportion accounted for by young users, retaining one wall and half the total.
A spatial area and a share of a total
占める (しめる) means to occupy or account for. The bookcase sentence concerns space, while the user sentence concerns a proportion of a total.
Attach 壁一面 and 全体の半数 to their subjects. The first is one whole wall of the room; the second is half of the users. Keep the spatial area and the population share distinct.
The bookcase's wall and young users' share
「書棚が部屋の壁一面を占めている。」 (A bookcase occupies an entire wall of the room.) The bookcase occupies one entire wall of the room. 一面 does not extend the occupied space to every wall or the whole room.
「若い利用者が全体の半数を占めた。」 (Young users accounted for half of the total.) Young users accounted for half of the total. 半数 gives the proportion, without a precise count of all users.
Retain the stated extent
The bookcase sentence describes continuing occupation of a wall with 占めている. The user sentence reports the share accounted for with 占めた. Read the object and action status together.
One whole wall and half of the users each have their own bounds. Neither becomes all the walls or a claim that every user was young.
What extents are occupied or accounted for in the two 占める examples?
Show the answer and why
The bookcase occupies one whole wall. Young users accounted for half of all users. No occupation of every wall or precise population count is supplied.