hAs riot ever said why the harrowing has gotten progressively smaller over the years

Smaller? It looks like it's been either getting bigger or it's been wildly fluctuating year after year.

This year, we have two new Harrowing skins (Underworld Wukong and Ravenborn LeBlanc), a new Harrowing featured game mode (Hexakill Twisted Treeline), new summoner icons, a video with the in-game lore behind the Harrowing, and even got promotional material before the Harrowing even started with shadows engulfing several champions on their champion pages.

Compare this to last year. One new skin, no Harrowing featured game mode, very little, if any, actual promo material, and did we get summoner icons? I don't remember.

If anything, this Harrowing looks more interesting than before.