rough, coarse, big
麤 centers on a lack of refinement: something rough, coarse, or large in a crude way. The three deer stacked together visually suggest something oversized and unpolished.
麤 is formed by tripling the deer component 鹿, likely to intensify the idea of roughness or coarseness associated with deer or large animals. The exact historical development is uncertain, but the tripling pattern is a known way to create emphatic characters.
Three deer crammed together look bulky and unrefined. Imagine a herd of deer stomping through a field, leaving everything rough and trampled.
For ソ, picture a coarse, rough piece of cloth being sewn (ソ) with thick thread. The sewing sound 'so' cues ソ while the rough fabric matches the meaning.