[Akababa]

If you look at it in terms of risk-adjusted returns (Sharpe ratio), a fixed transaction fee costs more Sharpe on a low-variance bet than it does on a high-variance bet. The variance of balanced contracts is higher than the variance of unbalanced ones (p(1-p) is concave with maximum at p=0.5), so after adjusting for risk the transaction fee is more significant for the unbalanced contract.

[nstj]

what’s a ‘balanced’ contract?

[virgilp]

I would be guessing that it's one where equal amounts of money are bet on each side/ there's a 50% probability of the result going either way.

[IfOnlyYouKnew]

A market where expected returns are 1, I. e. the implied probabilities sum to 100%