I also faced this issue, I fixed it by iterating to the attributes and reseting the old font size as follows
NSMutableAttributedString *res = [attributedText mutableCopy];
[res beginEditing];
[res enumerateAttribute:NSFontAttributeName
inRange:NSMakeRange(0, res.length)
options:0
usingBlock:^(id value, NSRange range, BOOL *stop) {
if (value) {
UIFont *oldFont = (UIFont *)value;
UIFont *newFont = [oldFont fontWithSize:15];
[res addAttribute:NSFontAttributeName value:newFont range:range];
}
}];
[res endEditing];
[self.textFileInputView setAttributedText:res];