Hack That Paralyzed US Health Care Turns Up Scrutiny on Insurer (1)

When a cyberattack on Change Healthcare paralyzed much of the US health-care system, some lawmakers saw it as proof its parent company, UnitedHealth Group Inc., was too big.

UnitedHealth Chief Executive Andrew Witty saw it differently. He has said that the company’s size kept the hack, which crippled a network that handled $2 trillion in health claims a year, from being more harmful. It was “important for the country that we own Change Healthcare,” Witty said earlier this month.

Witty is expected to appear at House and Senate hearings on Wednesday. He is likely to be asked whether UnitedHealth, which runs ...