Now boasting a $2.5 trillion market cap, a stock like Apple Inc. (NASDAQ: AAPL) would usually be considered too big to generate any exciting swings for a short-term trader.

The agency’s expected changes to U.S. stock-trading rules are likely to prompt pushback from the brokerages and market-making firms that handle small investors’ orders.

GameStop stock soars as online traders take on Wall Street funds 01:45. Wallstreetbets members have fought back by vowing never to sell, egging each other on with posts urging to buy more shares ...