“Bigger buttons are always better” ignores finite layout area

What it is

Fitts's law supports easier acquisition of a larger effective target when other conditions are equal. Screen space is finite, though: enlarging one button crowds other controls, content, or whitespace. “Bigger buttons are always better” is therefore not the law's conclusion, but a local extrapolation that ignores layout trade-offs.

Why it happens

A larger target expands endpoint tolerance while changing neighbours' width, spacing, and position. Enlarging everything reduces density and may add scrolling or hierarchy; enlarging one can speed a frequent path while harming a rarer one. Visual organisation also supports search and understanding, so space cannot be assigned only to motor efficiency.

Studying it

Compare alternative space allocations in the same task. Record acquisition, accidental activation, search time, scrolling, complete-task time, and information visibility across controls. Stratify by frequency and risk, not only speed on the enlarged target. Examine beneficiaries and harmed tasks instead of hiding regression in an overall average.

Where it stops holding

With abundant space or one high-risk action, a larger hit area has little downside and remains appropriate. Invisible expansion can avoid visual crowding but must not conflict with neighbours. Constraints change with screen, orientation, accessibility settings, and content length; no fixed size fits all.

Applying it

• Allocate space by frequency, consequence, and reach: enlarge important error-prone actions while protecting the minimum usable area of other essential actions.
• Prefer expanding real hit areas and spacing over indiscriminately making every visual control larger.
• Test small screens, long text, dynamic localisation, and assistive display settings to ensure enlargement has not pushed vital information out of view.

Related

• Same group: B1.11.1 The law assumes rapid aimed movement; slow tracking and dragging are outside its scope · B1.11.2 Coefficients must be calibrated for device, limb, and population · B1.11.3 The law covers movement time, not target search or deciding where to click · B1.11.4 Treating index of difficulty as a single quality score is misuse
• Nearby: B1.01 Fitts's law · B1.02 Fitts's law on touchscreens
• Search terms: Fitts's law · layout trade-off · target size · screen space