2305HCOOH \implies HCOO-+H+...(1)
CH3COOH \implies CH3COO-+H+....(2)
The +I effect of CH3 is more than that of H.
We know that +I effect destabilizes an anion.
Hence CH3COO- is less stable than HCOO- an readily undergoes the backward reaction.
Therefore the forward reaction involved in the dissociation of formic acid is more than that of acetic acid \impliesmore H+ ions are released in the 1st reaction making formic acid more acidic.
