The honest answer is that it depends on what you want your money to do.
A small charity feels a £50 donation. Costs are usually low, volunteers do more of the work, and you can often see the result locally. The trade-off is fragility: losing one grant or one key person can stop the service.
A large charity can fund multi-year research, campaign to change policy for far more people than it serves directly, and absorb an emergency without closing. Its overheads look higher because it employs the finance, safeguarding and audit functions that make that scale safe.
Neither is inherently better run. If you want to see well-scoring smaller charities, browse hidden gems; to weigh two options directly, use the comparison tool.