Well no, 5% is not a fixed number, it's relative to an original value and will be different when calculated from different values. 5% of a smaller number will always be less than 5% of a larger number and therefore whatever the two values (in this case the size of the toy and prop) 5% of each of those values will not be the same.
Here's the math:
For simplicity, we'll pretend that the real prop is 100 of a given unit of measurement long.
In the case where the toy is 5% smaller than the prop, we would take five percent of 100 and subtract that from the 100:
.05 * 100 = 5
100 - 5 = 95
However, in the case where the prop is 5% larger than the toy, five percent of the 95 length added to the 95 length doesn't give us 100:
.05 * 95 = 4.75
95 + 4.75 = 99.75
So if the prop, being 100 units, is truly 5% larger than the toy (not the other way around), the length of the toy(t) would have to be:
.05 * t + t = 100
.05t = 100 - 1t
1.05t = 100
t = 95.23809524