Brewers’ spent grains (BSGs) represent a significant by-product of beer production, yet their detailed compositional variation across brewery scales and beer technologies remains insufficiently explored. This study systematically characterized eleven BSG samples from Latvian breweries of diverse capacities and beer styles, assessing macronutrient profiles, amino acid content, phenolic compounds, and antioxidant activity. The results revealed considerable compositional heterogeneity: the moisture content ranged 71.9–83.9%, crude protein 17.8–30.83%, lipids 0.68–5.44%, and dietary fiber 59.61–71.64% of dry weight. All samples contained a complete set of essential amino acids, and the highest total phenolic content was observed in dark beer BSGs, which was up to 1.6 times higher than in light beer. Notably, a significant negative correlation was found between ABTS antioxidant activity and total phenolic content, indicating complex matrix interactions. These findings demonstrate a marked variation in BSG composition and underscore the impact of brewery parameters on nutritional and functional values, supporting efficient, tailored valorization strategies for functional food production.