That's about the size of it

村上春樹の "Pinball, 1973" を読んでいますが、私は想像力に欠けるので、どうも状況がピンときません。しかし我慢して読み続けます。
I had no idea what to do with the thing now that I'd caught it. Hind leg still pinned under the spring wire, the mouse died on the fourth morning. Seeing it lying there taught me a lesson. Everything needs an entrance and exit. That's about the size of it.
上の文は、入り口のある物は出口がある、もちろん例外もあって、その例の一つとして出てきたネズミ捕りの話の一部です。どうも話の内容にはまだついて行けませんが、私の関心はもっぱら英語の表現なので、この逸話に何の意味があるかは無視して、"That's about the size of it" の言い回しが気になりました。 似た様な表現が会話文にはありそうですが、思い出したのは５年前に取り上げた "That's the way to do it." だけです。この手の表現は英会話のテキストにはありそうですが、辞書で調べるのはやっかいな場合が多いのでどうかなと思いつつ辞書を見ると意外と簡単に説明が見つかりました。
・Macmillan Dictionary: spoken used for saying that you agree with someone's description or opinion of something: "So everyone's on strike?" "Yeah, that's about the size of it."
・Cambridge English Dictionary: used to show your agreement with someone who has said something correct: "So you mean you won't come to the party with me?" "Yes, that's about the size of it."
・Oxford English Dictionary: informal said to confirm someone’s assessment of a situation, especially of one regarded as bad:: "So if I drop my opposition to the road you’ll fix it so I don’t lose my job?’ ‘That’s about the size of it.’
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: Inf. That is the way it is. (Often a response to someone who has acknowledged bad news.) Bob: We only have a few hundred dollars left in the bank. Sally: That means that there isn't enough money for us to go on vacation? Bob: That's about the size of it. Bob: I'm supposed to take this bill to the county clerk's office and pay them four hundred dollars? Sally: That's about the size of it.