heriotza baino txiki txikiago den
The translation would be:
".... which is much smaller than death"
Note a typical basque construction: the repetition of the adjective to intensify it. I guess this resource is used in a lot of languages, but in basque it is used very frequently, also in syntactic situations in which is certain uncommon in other languages, as the very example that you bring up: a comparison. I don't know if an english speaking person would say "... small-smaller than death" instead of "much smaller than death". (Well, actually I don't know how frequently one would use the expression "smaller than death" at all...)
"argitasupean" breaks down into
Let' us take a look on the separate elements:
"argi": light (as adjective)
"tasu(n)": a suffix that indicates "quality"
for instance: "zuzen" means "right". "Zuzentasuna" is the quality of what is right: "justice"
"pe": it's the phonetic adaption of "behe", meaning "under"
a: the article
n: the inesive case: Non
so, all together: "argitasupean" -> "under the light"
... and yes: Urtz is a great singer