how did goku lose so much muscle

Goku didn’t really “lose” muscle in-universe so much as his design got leaner over time. The most likely reasons are an art-style shift, a move toward a more athletic look, and some scenes where he’s shown in a less bulky form for visual consistency.

What people usually mean

Across Dragon Ball’s run, Goku has gone from very bulky in early and mid- series art to more streamlined later on. Fans often notice this most in comparisons between older Dragon Ball Z-era frames and newer Dragon Ball Super-era designs.

Why it changed

• Art direction changed. Later designs favor a slimmer, cleaner silhouette rather than the huge bodybuilder look from earlier arcs.

• Training style can be read differently. Some fan discussions treat the leaner look as a shift from mass to efficiency, like an endurance/technique-focused fighter rather than a pure powerhouse.

• Certain forms and scenes exaggerate slimness. Some newer depictions make him look extra lean, which sparks the “where did the muscle go?” reaction online.

Quick takeaway

So the short answer is: it’s mostly a visual redesign, not a plot point. If you want, I can also break down which Dragon Ball eras made Goku look the bulkiest vs. the slimmest.