early naruto, aone, was always targetted at 172 or 175 megs. 172 megs is perfect to fit like 26 on a dvd, 175 is perfect for 4/cd but overflows the 26/dvd target by a couple megs. Many encoders these days aim for 170, to give more wiggle-room later in the series for particularly action-intensive episodes.

Some episodes get encoded at higher bitrates because they're more action-packed and faster motion.

Generally at sizes as small as 140 megs with as much action and as bright colors as Naruto has, you'll have visible video quality degradation, in the forms of blocking and artifacting in general. But then, 140 megs a year ago meant quite a bit more of a compromise in quality than 140 megs now means, because the codecs have matured more.

Every group has its own standards to follow. Some prefer smaller sizes over better video quality, and that's a tradeoff the encoder can make pretty easily. It has almost nothing to do with the subtitles themselves though.