Mechanism
At larger size you become your own problem: your orders move price and get worse fills. Liquidity is finite.
Your psychology changes too. You stop executing the same way when money feels real.
Many traders mistake a ‘scalable’ idea for a ‘profitable’ idea.
Microstructure note: stops fail most often at the same time liquidity disappears. That is not bad luck; it is structural. Your job is to avoid competing for fills in the worst queue.
- Prefer “don’t trade” windows over cleverness: rollover, open/close, data prints.
- Reduce size before you reduce stop distance. Size is the only lever that always works.
- Measure slippage by regime, not by average.
How it kills accounts
Increase size → execution degrades → performance drops → you tweak rules → overfit → lose confidence → abandon.
How it kills accounts:
- Edge looks fine in backtest.
- Live spreads widen at the exact wrong moments.
- Stops trigger inside noise, so you widen stops.
- Same size + wider stop = silent leverage increase.
- A normal spike becomes structural damage.
Rule that survives
Treat scaling as a separate project.
Increase size in steps and re-measure after costs.
If execution changes, strategy has changed.
Rule that survives:
- Spread is a gate, not a footnote. If it’s abnormal, you don’t trade or you trade smaller.
- Assume worst-case fills in fast markets.
- Size is the adapter: reduce size before changing the stop model.
Example archetype
You double size and suddenly your entries feel ‘worse’ and your exits slip. It’s not bad luck. You crossed a liquidity threshold.
Tell: if the trade only works when the spread is tight and price is smooth, it’s not an edge, it’s a regime bet.
Deep dive
Scaling is engineering
At size, you’re trading liquidity and impact. If you don’t model impact, you’re guessing.
Glossary: market impact, partial fill.
Field checklist
- Measure spread before entering. If it’s abnormal, you’re trading the wrong product at the wrong time.
- If volatility expands, reduce size first. Don’t “solve” it by widening stops with the same size.
- Avoid the predictable liquidity holes: rollover, session open/close, first minutes after data.
- Assume your stop may fill worse than your entry. Price the worst-case, not the brochure.
- If you cannot explain where liquidity comes from, trade smaller.