Grammar of nounの多さに

Question

品揃えの多さについつい買いこんでしまい、...
I ended up unintentionally buying ??? complete set...

I'm struggling to understand the part in bold. I know that 多い is 'many', and that you can convert adjectives to nouns by changing the last い to さ. So I get 多さ = 'manyness' (obviously not a real word) maybe 'multitude' or 'multiple'??

Now I have a 'multiple of complete sets' with a に on the end. I can't make any further sense of it.

I also wondered if when you add a さ the thing you make is a na-adjective rather than a noun so I could make an adverb from 多さに but that didn't help either.

I also notice there's no object marker を.

Answer 1

• 品揃え ≒ assortment/lineup of goods (Probably you can't use complete set here...)
• I think abundance is the closest noun to 多さ ≒ 豊富さ. 品揃えの多さ ≒ 品揃えの豊富さ means "the largeness of the selection of goods" or simply "the large selection of goods" in this context.
• 買い込む doesn't necessarily take an object. 買い込む by itself can mean "to do a lot of shopping".
• This に is a particle which can sometimes mark a cause/reason. (eg, そのニュースに驚いた, その言葉に嬉しくなった) In this case you can understand it as "because of (the abundance)" or "due to (the abundance)".