From my experience with Arakawa Under the Bridge (and it's been quite a bit of time), the two brothers (along with a number of other characters at Rec's welcoming party) don't get a proper introduction until later on in the first or second seasons.
I would say it was Shaft making an animation mistake, since the two of them weren't characters they were focusing on yet, so they could draw them without as much attention to detail/quality.
Perhaps it was something with the manga though; whether the author hadn't made a final decision on their appearances until later on, she just drew them like that for the time being. Shaft, for whatever reason, could've replicated this. (Part of me finds that unlikely because they would've received the characters' heights and such from the manga publishers by the time they started the anime, but part of me think it may be possible because it's Shaft lol.)
I could look through the manga and see what I could find, but right now, I'd say it was Shaft just making a mistake.